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poll_state_synchronize_rcu(mm->futex.phash.batches) is used by
futex_ref_drop() to check that a grace period has passed since the
current hash was published. This relies on batches referencing a grace
period which started after the hash pointer was assigned.
__futex_pivot_hash() sets mmph->batches before it replaces mmph->hash:
scoped_guard(rcu) {
mmph->batches = get_state_synchronize_rcu();
rcu_assign_pointer(mmph->hash, new);
}
The scoped_guard(rcu) doesn't stop new grace periods from starting, and
if one starts between those two assignments, futex_ref_drop() can move
forward while a reader still holds a pointer to the old hash.
Fix things by setting mmph->batches after assigning mmph->hash. The
scoped_guard(rcu) isn't needed, so let's drop that as well.
Fixes: 56180dd20c19 ("futex: Use RCU-based per-CPU reference counting instead of rcuref_t")
Assisted-by: kres
Signed-off-by: Chris Mason <mason@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://patch.msgid.link/20261001135022.2220288-1-mason@kernel.org
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git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull locking fixes from Ingo Molnar:
- Fix a softirq processing delay bug in local_interrupt_disable(),
which should mostly only affect the Rust runtime (Boqun Feng)
- Remove the hardirq_disable_count() function which caused the
previous bug and is now unused & unnecessary (Boqun Feng)
- lockdep: Invalidate stale class_cache entries for zapped classes
(Eric Dumazet)
- Fix rt_mutex specific futex scheduling helpers
(Sebastian Andrzej Siewior)
- Fix rcuwait use-after-free race during futex requeue PI (Yao Kai)
* tag 'locking-urgent-2026-09-06' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
futex: Prevent rcuwait use-after-free during requeue PI
futex: Provide rt_mutex_.*_schedule() equivalents for futex scheduling
locking/lockdep: Invalidate stale class_cache entries for zapped classes
preempt: Remove hardirq_disable_count()
interrupt: Disable interrupt before modifying hardirq_disable counter
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This is another run of the Coccinelle script for converting kmalloc()
family of allocations to kmalloc_obj() via the existing rules in
scripts/coccinelle/api/kmalloc_objs.cocci
This catches both the set of kmalloc() uses added since the first
kmalloc_obj() conversions in v7.0 and adds a large group missed in the
first pass due to Coccinelle not interacting well with the cleanup.h
scoped_...() family of macros[1]. I worked around this with spatch's
"--macro-file" argument to a file with all the scoped_...() macros mapped
to Coccinelle's YACFE_ITERATOR[2] as that was the closest viable control
flow indicator I could find.
Build tested allmodconfig on x86, arm64, arm, loongarch, mips, powerpc,
riscv, and s390 with no new warnings.
Link: https://lore.kernel.org/lkml/202609021314.8A9C0B8@keescook/ [1]
Link: https://github.com/coccinelle/coccinelle/blob/master/standard.h [2]
Signed-off-by: Kees Cook <kees+treewide@kernel.org>
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On PREEMPT_RT, FUTEX_CMP_REQUEUE_PI can trigger a KASAN report
(slab-out-of-bounds) in futex_requeue_pi_complete() invocation of
rcuwait_wake_up().
The futex_q used by futex_wait_requeue_pi() is allocated on the waiter's
stack. An early wakeup can race with a PI requeue as follows:
waiter requeue task
------ ------------
futex_wait_requeue_pi()
futex_do_wait()
schedule()
futex_requeue
futex_proxy_trylock_atomic()
futex_requeue_pi_prepare()
Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_IN_PROGRESS
* timeout/ signal wakes waiter *
futex_requeue_pi_wakeup_sync()
Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_WAIT
requeue_pi_wake_futex
futex_requeue_pi_complete()
cmpxchg Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_LOCKED
rcuwait_wait_event()
if (atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT)
break /* no schedule() */
/* q.pi_state->owner == current */
futex_private_hash_put()
/* return from syscall */
rcuwait_wake_up(&q->requeue_wait)
/* q is gone */
futex_requeue_pi_complete() publishes Q_REQUEUE_PI_LOCKED before
calling rcuwait_wake_up(). The waiter observes this state in
rcuwait_wait_event() before invoking schedule() in rcuwait_wait_event().
Here, the waiter is free leave the syscall before requeue task can
complete the wake.
To address this race skip rcuwait_wake_up() in the Q_REQUEUE_PI_LOCKED
case.
This state is only published by requeue_pi_wake_futex(), which saves
q->task before futex_requeue_pi_complete() and wakes the waiter via
wake_up_state().
This wake is intended to wake the waiter from its futex_do_wait() sleep.
If the waiter is still sleeping there, it can not get into the
Q_REQUEUE_PI_WAIT state (and require this removed wake).
Should the waiter be woken up from futex_do_wait() by other means (as in
this example) and sleep in futex_requeue_pi_wakeup_sync() then the
wake_up_state() from requeue_pi_wake_futex() will wake it, too.
Should the waiter task terminate before wake_up_state() had a chance to
wake the task then the task pointer does not become invalid because the
futex_hash_bucket::lock is held and the task pointer is RCU protected.
[bigeasy: Updated comment and commit message]
Fixes: 07d91ef510fb1 ("futex: Prevent requeue_pi() lock nesting issue on RT")
Signed-off-by: Yao Kai <yaokai34@huawei.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260901135453.3121948-3-bigeasy@linutronix.de
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There is rt_mutex_{pre|post}_schedule() around
rt_mutex_wait_proxy_lock() to ensure that sched_submit_work()/
sched_update_worker() is invoked before we schedule out and block on
rt_mutex while waiting for it become available.
The reason is that blocking on rt_mutex assigns a pi_waiter for the PI
chain and sched_submit_work() will also assign a pi_waiter if it blocks
on lock but a this point we already have a waiter assigned.
We can't skip sched_submit_work() entirely because I/O relies on the
fact that I/O queue is flushed while it blocks on a sleeping lock.
Therefore sched_submit_work() is moved before we block on the lock.
Sleeping lock in this context means mutex or rw_semaphore not spinlock_t
on PREEMPT_RT. Because the mutex abstraction on PREEMPT_RT uses the same
abstraction as the futex proxy lock, the futex code ended up using
rt_mutex_{pre|post}_schedule(), too.
Using it is/ was just to keep the task_struct::sched_rt_mutex assertion
happy. Futex proxy lock is used only in the syscall context of a task.
At this point it never got any I/O that needs to be flushed and it can't
be a workqueue that needs to notify that it will be scheduled out.
Therefore sched_submit_work() does nothing here.
By mistake futex_wait_requeue_pi() -> rt_mutex_wait_proxy_lock() did not
get the rt_mutex_{pre|post}_schedule() annotation. This was not noticed
because in this callchain the lock is (usually) not contended and so
rt_mutex_slowlock_block() does not schedule, triggering the assert.
Adding rt_mutex_pre_schedule() here looks wrong (as noted by PeterZ)
because at this point there is a pi_waiter recorded and invoking
sched_submit_work() with a possible lock contention would be wrong.
Add rt_mutex_futex_{pre|post}_schedule() which toggles the
sched_rt_mutex assert and does not involve sched_submit_work(). Add
asserts here to ensure that sched_submit_work() would do nothing. Use it
only in futex proxy lock case which is rt_mutex_wait_proxy_lock().
Remove it from futex_lock_pi().
Fixes: d14f9e930b90 ("locking/rtmutex: Use rt_mutex specific scheduler helpers")
Reported-by: Yao Kai <yaokai34@huawei.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260901135453.3121948-2-bigeasy@linutronix.de
Closes: https://lore.kernel.org/all/20260717084922.4153317-2-yaokai34@huawei.com
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git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull futex fixes from Ingo Molnar:
- Enforce that the private futex owner shares the mm when attaching
(Kyle Zeng, Thomas Gleixner)
- Fix race on the initial mm->futex.phash.ref allocation (Hyunwoo Kim)
- Fix might_sleep() warning in futex_pivot_pending() (Peter Zijlstra)
* tag 'locking-urgent-2026-08-22' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
futex: Fix might_sleep() warning in futex_pivot_pending()
futex: Fix race on the initial mm->futex.phash.ref allocation
futex: Clean up the redundant exit/exec functions
futex/pi: Plug private futex exec() race
futex: Sanitize and document task_struct::futex::state transitions
futex/pi: Reject cross-mm private futex owners
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A younger me put a WARN in might_sleep() to warn about nested sleep loops. This
younger me also build a wait-loop variant that can deal with it. This wait-loop
variant doesn't have all the fancy wrappers, since it isn't used much. It also
lacks wait-bit support.
Add the wait-bit support and use it to fix the nested wait issue.
Fixes: 8e7ff730dd96 ("futex: Fix race in futex_pivot_pending() during private hash resize")
Reported-by: syzbot+350a93852ac854927f45@syzkaller.appspotmail.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260820074927.GH1246887@noisy.programming.kicks-ass.net
Closes: https://syzkaller.appspot.com/bug?extid=350a93852ac854927f45
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git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull futex updates from Thomas Gleixner:
- Improvements to various futex self tests:
- Conversion to the selftest harness
- Provide and use thread creation and synchronization helpers to
reduce the dependency on delays, which tend to fail on loaded test
systems
- New tests for validating owner exit scenarios for robust and PI
futexes
- Runtime detect supported features and skip the tests if the kernel
has no support
- A few minor fixes
* tag 'locking-futex-2026-08-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
selftests/futex: Give circular-list nodes static storage
selftests/futex: Use thread synchronization helpers instead of usleep()
selftests/futex: Provide thread creation and synchronization helpers
selftests/futex: Dynamically skip unsupported tests
selftests/futex: Add FUTEX_LOCK_PI owner-exiting coverage
selftests/futex: Migrate robust_list to harness
selftests/futex: Migrate futex_priv_hash to harness
selftests/futex: Migrate futex_numa_mpol to harness
selftests/futex: Migrate futex_requeue_pi_signal_restart to harness
selftests/futex: Migrate futex_requeue_pi_mismatched_ops to harness
selftests/futex: Migrate futex_requeue_pi to harness
selftests/futex: Migrate futex_requeue to harness
selftests/futex: Migrate futex_wait_uninitialized_heap to harness
selftests/futex: Migrate futex_wait_private_mapped_file to harness
selftests/futex: Migrate futex_wait to harness
selftests/futex: Correct validation logic in waitv
selftests/futex: Migrate functional tests to harness
selftests/futex: Remove static keyword from 'head'
futex: Remove unnecessary NULL check before kvfree()
selftests/rseq: Replace glibc-specific __GNUC_PREREQ with portable check
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git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull locking updates from Ingo Molnar:
"Futexes:
- Use runtime constants for futex_hash computation (K Prateek Nayak,
Peter Zijlstra)
- Optimise the size check get_futex_key() (Sebastian Andrzej Siewior)
- Avoid private hash use-after-free on final put (Felix Hoffmann)
- Tell kmemleak we're not leaking __futex_queues (Peter Zijlstra)
Rust integration updates:
- Implement refcounted interrupt disable and SpinLockIrq for Rust
(Boqun Feng, Heiko Carstens, Joel Fernandes, Lyude Paul)
- Rust sync: add helpers for mb, dma_mb and friends; add generic
memory barriers and use LKMM atomics instead of Rust atomics in the
revocable code (Gary Guo)
- Add abstraction and integrate synchronize_rcu() (Philipp Stanner)
Lock debugging:
- Add qspinlock contended_release tracepoint (Dmitry Ilvokhin, Peter
Zijlstra)
- Enable the printing of held locks of remote running tasks and print
task CPU (Ingo Molnar)
- percpu-rwsem: Annotate intentional data race in readers_active_check()
(Sun Shaojie)
Misc fixes and updates by Boqun Feng, Peter Zijlstra, Fangrui Song,
Naveen Kumar Chaudhary and Thomas Huth"
* tag 'locking-core-2026-08-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (44 commits)
rust: sync: Introduce SpinLockIrq::lock_with() and friends
rust: sync: Add SpinLockIrq
rust: sync: Use super::* in spinlock.rs
rust: helper: Add spin_{un,}lock_irq_{enable,disable}() helpers
rust: Introduce interrupt module
s390/preempt: Enable HAS_SEPARATE_PREEMPT_RESCHED_BITS
arm64: sched/preempt: Enable HAS_SEPARATE_PREEMPT_RESCHED_BITS
preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS
sched: Avoid signed comparison of preempt_count() in __cant_migrate()
sched: Remove the unused preempt_offset parameter of __cant_sleep()
locking: Switch to _irq_{disable,enable}() variants in cleanup guards
irq: Add KUnit test for refcounted interrupt enable/disable
irq,spin_lock: Add counted interrupt disabling/enabling
openrisc: Include <linux/cpumask.h> in smp.h
preempt: Introduce __preempt_count_{sub,add}_return()
preempt: Introduce HARDIRQ_DISABLE_BITS
preempt: Track NMI nesting to separate per-CPU counter
futex: Tell kmemleak we're not leaking __futex_queues
x86/paravirt: Trace contended_release on unlock
tracing/lock: Use TRACE_EVENT_FN() for contended_release
...
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futex_hash_allocate() allocates mm->futex.phash.ref without any locking.
Commit d9b05321e21e ("futex: Move futex_hash_free() back to __mmput()")
moved the allocation here and assumed that the process has just a single
thread at this point.
Commit ee9dce44362b ("futex: Drop CLONE_THREAD requirement for private
default hash alloc") widened need_futex_hash_allocate_default() to cover
any CLONE_VM clone, but left out vfork because the parent is suspended and
cannot race.
That no longer holds once vfork is nested. If a vfork child calls vfork
again and is then killed with SIGKILL, the parent is released from its
vfork wait and runs concurrently with the grandchild in the same mm.
Neither of them went through futex_hash_allocate_default().
When both call prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS) at the same
time, each one sees mm->futex.phash.ref as NULL and stores its own percpu
counter. Only the last store survives. The counter stored first is no
longer reachable from the mm, so the references on it are not seen by
__futex_ref_atomic_end(). A private hash that still has references is then
considered dead and freed, and a task that still holds one of its buckets
writes into freed memory in futex_q_lock().
Store the counter once with cmpxchg() and let the loser free_percpu() its
own. The initial reference has to be taken before the store, otherwise
another task can install a private hash while the counter is still 0.
Fixes: d9b05321e21e ("futex: Move futex_hash_free() back to __mmput()")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/ansrpP4ImE1MaBY9@v4bel
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futex_exit_release() and futex_exec_release() are identical now. That means
also exit_mm_release() and exec_mm_release() are identical.
Consolidate the whole lot and remove the redundant copies.
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Kyle Zeng <kylebot@openai.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
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The check for private futexes whether the waiter's mm, which is stored in
the futex_key and copied into the pi_state, is the same as the owner's mm
is not sufficient for exec(). exec() has a gap where the mm check fails to
give the correct answer:
exec()
...
exec_release_mm()
futex_exec_release()
tsk::futex::exit_state = EXITING;
cleanup_robust_list();
1) tsk::futex::exit_state = OK;
...
old_mm = tsk::mm;
2) tsk::mm = ->mm;
Between #1 and #2 the check for the mm is wrong as that mm is about to be
swapped out and eventually freed.
Plug this gap by:
1) Setting tsk::futex::exit_state to FUTEX_STATE_DEAD in
futex_exec_release()
2) Setting tsk::futex::exit_state to FUTEX_STATE_OK after
the mm has been switched.
From a futex point of view the task is dead after it finished the robust
list cleanup up to the point where it sets the state to OK again.
Fixes: 80367ad01d93 ("futex: Add basic infrastructure for local task local hash")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Kyle Zeng <kylebot@openai.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
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The futex state is used to prevent a waiter from attaching to the lock
owner while the owner runs the futex cleanup in exit() or exec().
Only the state transition from FUTEX_STATE_OK to FUTEX_STATE_EXITING must
be done with the task's pi_lock held, the transition away from
FUTEX_STATE_EXITING has no serialization requirements on the writer side,
but it's completely non obvious why. It's magically protected by
exit_pi_state(), which operates under tsk::pi_lock, as that's the state
which has to be correct when the waiter observes the new state.
OTOH, taking the pi_lock in futex_cleanup_end() is not a performance issue
because at that point the lock should be uncontended in the vast majority
of cases.
Aside of that the handling of FUTEX_STATE_EXITING in attach_to_pi_owner()
and handle_exit_race() is confusing at best.
Protect the store in futex_cleanup_end() with tsk::pi_lock, handle
FUTEX_STATE_EXITING in attach_to_pi_owner() explicitly and document how
this is supposed to work.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Kyle Zeng <kylebot@openai.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
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A private futex key borrows the waiter's mm without taking an mm_users
reference. Nevertheless, attach_to_pi_owner() currently accepts an owner
from a different address space and copies the private key into the owner's
PI state.
When that owner exits, exit_pi_state_list() uses the saved key to find the
hash bucket and acquires a reference to the waiter's private hash. If the
last user of the waiter's mm exits concurrently, futex_hash_free() frees
the hash while the owner still uses its bucket and reference.
Prevent this by validating in attach_to_pi_owner() that, for private
futexes, the owner mm and waiter mm are the same. Perform the check with
the owner's pi_lock held and after validating owner::futex::state to
serialize against a concurrent PI-state exit cleanup.
[ tglx: Amended comment ]
Fixes: 80367ad01d93 ("futex: Add basic infrastructure for local task local hash")
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Assisted-by: Codex:gpt-5.6-sol
Cc: stable@vger.kernel.org
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Kmemleak doesn't know about runtime_const stuff and figures we're leaking
__futex_queues. So add this little annotation to tell it all is well.
Fixes: b78b0b658252 ("futex: Use runtime constants for __futex_hash() hot path")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202608071053.6db6276e-lkp@intel.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260807152353.GP687043@noisy.programming.kicks-ass.net
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A task performing a custom private hash resize can remain blocked in
uninterruptible sleep indefinitely. The hung-task detector reports:
INFO: task futex-resizer:314 blocked for more than 10 seconds.
task:futex-resizer state:D stack:14824 pid:314 tgid:312 ppid:311
Call Trace:
__schedule+0x521/0xf30
schedule+0x22/0xa0
futex_hash_allocate+0x3db/0x490
__do_sys_prctl+0x6f5/0xbd0
do_syscall_64+0xf9/0x530
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Kernel panic - not syncing: hung_task: blocked tasks
futex_pivot_pending() allows the resize request to continue when
either no replacement hash is pending (hash_new == NULL) or the current
hash reference count has reached zero.
After the final-reference wake, another futex task can complete the
pivot between the two observations:
T1 T2
futex_hash_allocate()
wait_var_event(mm, ...)
futex_pivot_pending(mm)
hash_new != NULL
futex_hash()
futex_ref_get(old) -> false
futex_pivot_hash(mm)
hash_new = NULL
__futex_pivot_hash(mm, new)
rcu_assign_pointer(hash, new)
fph = rcu_dereference(hash) /* new */
futex_ref_is_dead(fph) -> false
schedule()
The pivot changes the state from hash_new != NULL with a dead current
hash to hash_new == NULL with a live current hash. Because
futex_pivot_pending() reads hash_new and hash without serialization,
the resize task can observe hash_new in the pre-pivot state and hash in
the post-pivot state, causing futex_pivot_pending() to return false even
though the pivot has completed. The task then goes to sleep after the
wakeup has already been consumed.
Serialize state reads in futex_pivot_pending() using futex_mm_phash::lock.
This guarantees that futex_pivot_pending() observes hash_new and hash
atomically, eliminating the race condition.
Fixes: bd54df5ea7ca ("futex: Allow to resize the private local hash")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Yao Kai <yaokai34@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260804125530.3933754-1-yaokai34@huawei.com
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futex_private_hash_put() drops the reference to fph before evaluating
fph->mm for wake_up_var(). futex_ref_put() enables preemption again before
returning. If that put drops the final reference and the task is preempted,
another task can pivot to the replacement hash and free the old hash after
an RCU grace period. The first task then reads fph->mm from the freed
allocation when it resumes.
KASAN reports a slab-use-after-free in futex_private_hash_put(), with the
read at offset 24 in a freed kmalloc-512 allocation. The allocation and
free stacks point to futex_hash_allocate() and the RCU free path,
respectively.
Load the mm pointer while the fph reference is still held and pass the
saved value to wake_up_var(). wake_up_var() uses the pointer as a waitqueue
key and does not dereference the mm through it.
Fixes: bd54df5ea7ca ("futex: Allow to resize the private local hash")
Signed-off-by: Felix Hoffmann <f3lix.dev@gmx.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260731155024.1150011-1-f3lix.dev@gmx.de
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A robust futex unlock stores 0 over the whole futex value - wiping
FUTEX_WAITERS - and wakes a single waiter. That wakeup is a one-shot
notification: the protocol relies on its recipient to either acquire the
futex (and eventually unlock while aware of the remaining contention) or
re-arm FUTEX_WAITERS before sleeping again. If the woken waiter is killed
before it can do either, the kernel must jump in and wake the next task
down the line.
This is a known complication of the futex protocol with a previous
partial fix in commit ca16d5bee598 ("futex: Prevent robust futex exit
race"). Unfortunately, that fix is insufficient.
If a third task re-acquired the futex through the uncontended fast
path in the meantime, the notification is lost: robust exit processing
sees that it is owned by another task and does nothing, while the new
owner sees no FUTEX_WAITERS when it unlocks and wakes nobody.
The remaining waiters sleep forever behind a free futex:
A owns the futex, B and C sleep in FUTEX_WAIT
uval == A | FUTEX_WAITERS
A robust unlock: store 0, FUTEX_WAKE(1) wakes B
uval == 0
D fast path acquire: cmpxchg(0 -> D)
uval == D, no FUTEX_WAITERS
B killed before acting on the wakeup
B exit walk, pending op: owner D != B -> no action
D unlock: no FUTEX_WAITERS -> no wake
C sleeps forever
This is clearly a shortcoming in the implementation, which fails to keep
the FUTEX_WAITERS bit consistent.
Work around this by augmenting the robust list exit processing to also
perform the extra wakeup if the futex word is owned by another thread but
FUTEX_WAITERS is not set.
This does not fix the problem of a non-contended take over/release and free
sequence, which has been discussed for years and has been addressed by
commit 3ca9595d9fb6 ("futex: Add support for unlocking robust futexes") and
subsequent changes, but failed to take the problem described above into
account.
A more complete solution which is based on the in kernel unlock of
contended robust futexes has been discussed in the context of this change
and should show up in mainline sooner than later.
[ tglx: Amend change log slightly and fixup coding style ]
Fixes: ca16d5bee598 ("futex: Prevent robust futex exit race")
Signed-off-by: Keno Fischer <keno@juliahub.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Assisted-by: ClaudeCode:claude-fable-5 tla+
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260730194705.38981-1-keno@juliacomputing.com
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The futex address must be naturally aligned and this is checked via
"address % size" where `address' is the supplied address and `size' is
the expected size of futex. It is guaranteed that `size' is power of two
but the compiler does not see it and creates here a `div' operation
(x86, arm, gcc-15).
We can take advantage of the pow2 property and rewrite it as
"address & (size-1)".
As per testing, the command
|perf bench futex hash -f 1 -b 16384 -t 1 -r 30
improved from
| [thread 0] futex: 0x5619f931f740 [ 7001583 ops/sec ]
to
| [thread 0] futex: 0x55da173e5740 [ 7376137 ops/sec ]
or by 5.3%
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260701161736.xYYizA0e@linutronix.de
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Runtime constify the read-only after init data __futex_shift(shift_32),
__futex_mask(mask_32), and __futex_queues(ptr) used in __futex_hash()
hot path to avoid referencing global variable.
This also allows __futex_queues to be allocated dynamically to
"nr_node_ids" slots instead of reserving config dependent MAX_NUMNODES
(1 << CONFIG_NODES_SHIFT) worth of slots upfront.
Runtime constants are initialized before their first access and
runtime_const_init() provides necessary barrier to ensure subsequent
accesses are not reordered against their initialization.
No functional changes intended.
perf bench futex on a 3rd Gen EPYC (2 x 64C/128T):
+----------------+-----------+-----------+-----------+--------------+
| Benchmark | Kernel 1 | Kernel 2 | Unit | % Improvement|
| | (avg/5) | (avg/5) | | (K2 vs K1) |
+----------------+-----------+-----------+-----------+--------------+
| Wake-parallel | 0.01614 | 0.00456 | ms | +71.75% |
| Requeue | 0.26394 | 0.24644 | ms | +6.63% |
| Lock-pi | 34.0 | 57.2 | ops/sec | +68.24% |
+----------------+-----------+-----------+-----------+--------------+
Performance testing on a 144-thread Intel(R) Xeon(R) CPU E7-8890 v3 (4 NUMA nodes):
+-------------------------------------------------------------+
| perf bench futex hash -b 0 |
+----------------------+------------+------------+------------+
| Configuration | As-is | Patched | Delta |
+----------------------+------------+------------+------------+
| 1 thread, 1 futex | 6,449,632 | 6,532,004 | +1.28% |
| 144 threads, 1024 fx | 2,111,486 | 2,139,685 | +1.34% |
+----------------------+------------+------------+------------+i
[ prateek: Dynamically allocate __futex_queues, mark the global data
__ro_after_init since they are constified after futex_init(). ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> # MAX_NUMNODES bloat
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Tested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Link: https://patch.msgid.link/20260227161841.GH606826@noisy.programming.kicks-ass.net
Link: https://patch.msgid.link/20260728052540.4728-9-kprateek.nayak@amd.com
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The NULL check before kvfree() is unnecessary and triggers a Coccinelle
warning, since kvfree() handles NULL arguments internally.
Reported by ifnullfree.cocci Coccinelle semantic patch script.
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260630133530.2577832-1-vasilisalmpanis@gmail.com
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on self-deadlock""
The commit cited below should not have been merged. It attemted to fix an
existing problem ansd thereby introduced new problems by keeping the
pi_state in state Q_REQUEUE_PI_IN_PROGRESS and leaking it.
Based on the commit description the intention was to handle the case
when task_blocks_on_rt_mutex() returns -EDEADLK and the following
remove_waiter() dereferences the NULL pointer in waiter->task.
That is already handled by Davidlohr in commit 40a25d59e85b3
("locking/rtmutex: Skip remove_waiter() when waiter is not enqueued") and
requires no further acting.
Revert the commit breaking the "waiter == owner" case again.
Fixes: 74e144274af39 ("futex/requeue: Prevent NULL pointer dereference in remove_waiter() on self-deadlock")
Reported-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260701131150.0Ijhq4Dw@linutronix.de
Closes: https://lore.kernel.org/all/20260629020049.2082397-1-michael.bommarito@gmail.com
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gitolite.kernel.org:pub/scm/linux/kernel/git/tip/tip
Pull locking updates from Ingo Molnar:
"Futex updates:
- Optimize futex hash bucket access patterns (Peter Zijlstra)
- Large series to address the robust futex unlock race for real, by
Thomas Gleixner:
"The robust futex unlock mechanism is racy in respect to the
clearing of the robust_list_head::list_op_pending pointer because
unlock and clearing the pointer are not atomic.
The race window is between the unlock and clearing the pending op
pointer. If the task is forced to exit in this window, exit will
access a potentially invalid pending op pointer when cleaning up
the robust list.
That happens if another task manages to unmap the object
containing the lock before the cleanup, which results in an UAF.
In the worst case this UAF can lead to memory corruption when
unrelated content has been mapped to the same address by the time
the access happens.
User space can't solve this problem without help from the kernel.
This series provides the kernel side infrastructure to help it
along:
1) Combined unlock, pointer clearing, wake-up for the
contended case
2) VDSO based unlock and pointer clearing helpers with a
fix-up function in the kernel when user space was interrupted
within the critical section.
... with help by André Almeida:
- Add a note about robust list race condition (André Almeida)
- Add self-tests for robust release operations (André Almeida)
Context analysis updates:
- Implement context analysis for 'struct rt_mutex'. (Bart Van Assche)
- Bump required Clang version to 23 (Marco Elver)
Guard infrastructure updates:
- Series to remove NULL check from unconditional guards (Dmitry
Ilvokhin)
Lockdep updates:
- Restore self-test migrate_disable() and sched_rt_mutex state on
PREEMPT_RT (Karl Mehltretter)
Membarriers updates:
- Use per-CPU mutexes for targeted commands (Aniket Gattani)
- Modernize membarrier_global_expedited with cleanup guards (Aniket
Gattani)
- Add rseq stress test for CFS throttle interactions (Aniket Gattani)
percpu-rwsems updates:
- Extract __percpu_up_read() to optimize inlining overhead (Dmitry
Ilvokhin)
Seqlocks updates:
- Allow UBSAN_ALIGNMENT to fail optimizing (Heiko Carstens)
Lock tracing:
- Add contended_release tracepoint to sleepable locks such as
mutexes, percpu-rwsems, rtmutexes, rwsems and semaphores (Dmitry
Ilvokhin)
MAINTAINERS updates:
- MAINTAINERS: Add RUST [SYNC] entry (Boqun Feng)
Misc updates and fixes by Randy Dunlap, YE WEI-HONG, Fabricio Parra,
Dmitry Ilvokhin and Peter Zijlstra"
* tag 'locking-core-2026-06-14' of gitolite.kernel.org:pub/scm/linux/kernel/git/tip/tip: (36 commits)
locking: Add contended_release tracepoint to sleepable locks
locking/percpu-rwsem: Extract __percpu_up_read()
tracing/lock: Remove unnecessary linux/sched.h include
futex: Optimize futex hash bucket access patterns
rust: sync: completion: Mark inline complete_all and wait_for_completion
MAINTAINERS: Add RUST [SYNC] entry
cleanup: Specify nonnull argument index
selftests: futex: Add tests for robust release operations
Documentation: futex: Add a note about robust list race condition
x86/vdso: Implement __vdso_futex_robust_try_unlock()
x86/vdso: Prepare for robust futex unlock support
futex: Provide infrastructure to plug the non contended robust futex unlock race
futex: Add robust futex unlock IP range
futex: Add support for unlocking robust futexes
futex: Cleanup UAPI defines
x86: Select ARCH_MEMORY_ORDER_TSO
uaccess: Provide unsafe_atomic_store_release_user()
futex: Provide UABI defines for robust list entry modifiers
futex: Move futex related mm_struct data into a struct
futex: Make futex_mm_init() void
...
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Breno reported significant c2c HITM in a futex hash heavy workload.
It turns out that the hash bucket to private hash table reverse pointer
(futex_hash_bucket::priv) was to blame. Notably when the hash buckets are
heavily contended, the: 'fph = bh->priv;' load in futex_hash() will typically
miss and consequently become quite expensive.
Since this load in particular is quite superfluous, removing it is fairly
straight forward. However, removing it does not in fact achieve anything much.
The pain moves to the next user, notably: futex_hash_put().
Therefore rework the whole private hash refcounting to avoid needing this back
pointer (and removing it). Instead of passing around 'struct futex_hash_bucket
*hb', pass around a new structure that contains it and the related 'struct
futex_private_hash *fph' pointer in tandem.
Funnily this turns out to remove more code than it adds and significantly
improves futex hash performance (as measured by 'perf bench futex hash'):
SKL dual socket 112 threads:
Baseline Patched
shared (16k) 1571857 1641435 + 4.4%
autosize (512) 646390 903371 +39.7%
-b 256 464395 587014 +26.4%
-b 512 715687 995943 +39.2%
-b 1024 995085 1396328 +40.3%
-b 2048 1293114 1668395 +29.0%
-b 4096 2124438 2240228 + 5.5%
Zen3 dual socket 256 threads:
Baseline Patched
shared (16k) 1275840 1381279 + 8.2%
autosize (512) 1252745 1482179 +18.3%
-b 256 856274 955455 +11.5%
-b 512 1267490 1544010 +21.8%
-b 1024 1424013 1625424 +14.1%
-b 2048 1505181 1669342 +10.9%
-b 4096 1465993 1688932 +15.2%
AMD EPYC 9D64 (Zen4, single socket) 176 threads:
Baseline Patched Delta
shared (16k) 1,230,599 1,368,655 +11.2%
autosize (1024) 1,285,440 1,556,946 +21.1%
-b 256 1,341,471 1,520,303 +13.3%
-b 512 1,438,330 1,599,319 +11.2%
-b 1024 1,443,772 1,622,493 +12.4%
-b 2048 1,472,108 1,643,975 +11.7%
-b 4096 1,333,098 1,570,897 +17.8%
Reported-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Breno Leitao <leitao@debian.org>
Tested-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260610135510.GB1430057@noisy.programming.kicks-ass.net
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When the FUTEX_ROBUST_UNLOCK mechanism is used for unlocking (PI-)futexes,
then the unlock sequence in user space looks like this:
1) robust_list_set_op_pending(mutex);
2) robust_list_remove(mutex);
lval = gettid();
3) if (atomic_try_cmpxchg(&mutex->lock, lval, 0))
4) robust_list_clear_op_pending();
else
5) sys_futex(OP | FUTEX_ROBUST_UNLOCK, ....);
That still leaves a minimal race window between #3 and #4 where the mutex
could be acquired by some other task, which observes that it is the last
user and:
1) unmaps the mutex memory
2) maps a different file, which ends up covering the same address
When then the original task exits before reaching #5 then the kernel robust
list handling observes the pending op entry and tries to fix up user space.
In case that the newly mapped data contains the TID of the exiting thread
at the address of the mutex/futex the kernel will set the owner died bit in
that memory and therefore corrupt unrelated data.
On X86 this boils down to this simplified assembly sequence:
mov %esi,%eax // Load TID into EAX
xor %ecx,%ecx // Set ECX to 0
#3 lock cmpxchg %ecx,(%rdi) // Try the TID -> 0 transition
.Lstart:
jnz .Lend
#4 movq %rcx,(%rdx) // Clear list_op_pending
.Lend:
If the cmpxchg() succeeds and the task is interrupted before it can clear
list_op_pending in the robust list head (#4) and the task crashes in a
signal handler or gets killed then it ends up in do_exit() and subsequently
in the robust list handling, which then might run into the unmap/map issue
described above.
This is only relevant when user space was interrupted and a signal is
pending. The fix-up has to be done before signal delivery is attempted
because:
1) The signal might be fatal so get_signal() ends up in do_exit()
2) The signal handler might crash or the task is killed before returning
from the handler. At that point the instruction pointer in pt_regs is
not longer the instruction pointer of the initially interrupted unlock
sequence.
The right place to handle this is in __exit_to_user_mode_loop() before
invoking arch_do_signal_or_restart() as this covers obviously both
scenarios.
As this is only relevant when the task was interrupted in user space, this
is tied to RSEQ and the generic entry code as RSEQ keeps track of user
space interrupts unconditionally even if the task does not have a RSEQ
region installed. That makes the decision very lightweight:
if (current->rseq.user_irq && within(regs, csr->unlock_ip_range))
futex_fixup_robust_unlock(regs, csr);
futex_fixup_robust_unlock() then invokes a architecture specific function
to return the pending op pointer or NULL. The function evaluates the
register content to decide whether the pending ops pointer in the robust
list head needs to be cleared.
Assuming the above unlock sequence, then on x86 this decision is the
trivial evaluation of the zero flag:
return regs->eflags & X86_EFLAGS_ZF ? regs->dx : NULL;
Other architectures might need to do more complex evaluations due to LLSC,
but the approach is valid in general. The size of the pointer is determined
from the matching range struct, which covers both 32-bit and 64-bit builds
including COMPAT.
The unlock sequence is going to be placed in the VDSO so that the kernel
can keep everything synchronized, especially the register usage. The
resulting code sequence for user space is:
if (__vdso_futex_robust_list$SZ_try_unlock(lock, tid, &pending_op) != tid)
err = sys_futex($OP | FUTEX_ROBUST_UNLOCK,....);
Both the VDSO unlock and the kernel side unlock ensure that the pending_op
pointer is always cleared when the lock becomes unlocked.
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.773669210@kernel.org
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There will be a VDSO function to unlock robust futexes in user space. The
unlock sequence is racy vs. clearing the list_pending_op pointer in the
tasks robust list head. To plug this race the kernel needs to know the
instruction window. As the VDSO is per MM the addresses are stored in
mm_struct::futex.
Architectures which implement support for this have to update these
addresses when the VDSO is (re)mapped and indicate the pending op pointer
size which is matching the IP.
Arguably this could be resolved by chasing mm->context->vdso->image, but
that's architecture specific and requires to touch quite some cache
lines. Having it in mm::futex reduces the cache line impact and avoids
having yet another set of architecture specific functionality.
To support multi size robust list applications (gaming) this provides two
ranges when COMPAT is enabled.
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.718926819@kernel.org
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Unlocking robust non-PI futexes happens in user space with the following
sequence:
1) robust_list_set_op_pending(mutex);
2) robust_list_remove(mutex);
lval = 0;
3) lval = atomic_xchg(lock, lval);
4) if (lval & WAITERS)
5) sys_futex(WAKE,....);
6) robust_list_clear_op_pending();
That opens a window between #3 and #6 where the mutex could be acquired by
some other task which observes that it is the last user and:
A) unmaps the mutex memory
B) maps a different file, which ends up covering the same address
When the original task exits before reaching #6 then the kernel robust list
handling observes the pending op entry and tries to fix up user space.
In case that the newly mapped data contains the TID of the exiting thread
at the address of the mutex/futex the kernel will set the owner died bit in
that memory and therefore corrupting unrelated data.
PI futexes have a similar problem both for the non-contented user space
unlock and the in kernel unlock:
1) robust_list_set_op_pending(mutex);
2) robust_list_remove(mutex);
lval = gettid();
3) if (!atomic_try_cmpxchg(lock, lval, 0))
4) sys_futex(UNLOCK_PI,....);
5) robust_list_clear_op_pending();
Address the first part of the problem where the futexes have waiters and
need to enter the kernel anyway. Add a new FUTEX_ROBUST_UNLOCK flag, which
is valid for the sys_futex() FUTEX_UNLOCK_PI, FUTEX_WAKE, FUTEX_WAKE_BITSET
operations.
This deliberately omits FUTEX_WAKE_OP from this treatment as it's unclear
whether this is needed and there is no usage of it in glibc either to
investigate.
For the futex2 syscall family this needs to be implemented with a new
syscall.
The sys_futex() case [ab]uses the @uaddr2 argument to hand the pointer to
robust_list_head::list_pending_op into the kernel. This argument is only
evaluated when the FUTEX_ROBUST_UNLOCK bit is set and is therefore backward
compatible.
This is an explicit argument to avoid the lookup of the robust list pointer
and retrieving the pending op pointer from there. User space has the
pointer already available so it can just put it into the @uaddr2
argument. Aside of that this allows the usage of multiple robust lists in
the future without any changes to the internal functions as they just operate
on the provided pointer.
This requires a second flag FUTEX_ROBUST_LIST32 which indicates that the
robust list pointer points to an u32 and not to an u64. This is required
for two reasons:
1) sys_futex() has no compat variant
2) The gaming emulators use both both 64-bit and compat 32-bit robust
lists in the same 64-bit application
As a consequence 32-bit applications have to set this flag unconditionally
so they can run on a 64-bit kernel in compat mode unmodified. 32-bit
kernels return an error code when the flag is not set. 64-bit kernels will
happily clear the full 64 bits if user space fails to set it.
In case of FUTEX_UNLOCK_PI this clears the robust list pending op when the
unlock succeeded. In case of errors, the user space value is still locked
by the caller and therefore the above cannot happen.
In case of FUTEX_WAKE* this does the unlock of the futex in the kernel and
clears the robust list pending op when the unlock was successful. If not,
the user space value is still locked and user space has to deal with the
returned error. That means that the unlocking of non-PI robust futexes has
to use the same try_cmpxchg() unlock scheme as PI futexes.
If the clearing of the pending list op fails (fault) then the kernel clears
the registered robust list pointer if it matches to prevent that exit()
will try to handle invalid data. That's a valid paranoid decision because
the robust list head sits usually in the TLS and if the TLS is not longer
accessible then the chance for fixing up the resulting mess is very close
to zero.
The problem of non-contended unlocks still exists and will be addressed
separately.
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.670514505@kernel.org
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The marker for PI futexes in the robust list is a hardcoded 0x1 which lacks
any sensible form of documentation.
Provide proper defines for the bit and the mask and fix up the usage
sites. Thereby convert the boolean pi argument into a modifier argument,
which allows new modifier bits to be trivially added and conveyed.
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.458758556@kernel.org
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