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futex: Acquire a hash reference in futex_wait_multiple_setup()

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futex_wait_multiple_setup() changes task_struct::__state to
!TASK_RUNNING and then enqueues on multiple futexes. Every
futex_q_lock() acquires a reference on the global hash which is
dropped later.

If a rehash is in progress then the loop will block on
mm_struct::futex_hash_bucket for the rehash to complete and this will
lose the previously set task_struct::__state.

Acquire a reference on the local hash to avoiding blocking on
mm_struct::futex_hash_bucket.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-9-bigeasy@linutronix.de
This commit is contained in:
Sebastian Andrzej Siewior 2025-04-16 18:29:08 +02:00 • committed by Peter Zijlstra
parent d854e4e785
commit 3f6b233018
1 changed files with 6 additions and 0 deletions
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6
kernel/futex/waitwake.c
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@ -406,6 +406,12 @@ int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *woken)
int ret, i;
u32 uval;
/*
* Make sure to have a reference on the private_hash such that we
* don't block on rehash after changing the task state below.
*/
guard(private_hash)();
/*
* Enqueuing multiple futexes is tricky, because we need to enqueue
* each futex on the list before dealing with the next one to avoid