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mm/filemap: don't lose a folio lock wakeup to a signal

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An exclusive waiter in folio_wait_bit_common() can leave the wait loop
because of a fatal signal, then be woken by folio_unlock() before
finish_wait(). If the waker did not hand over the lock, the waiter returns
-EINTR without trying to acquire it.

folio_wake_bit() stops after waking the first exclusive waiter, so this
loses the wakeup and can leave other waiters asleep indefinitely on an
unlocked folio.

After finish_wait(), let a waiter woken without a lock handoff try once
more to acquire the lock. If successful, return success so the caller's
eventual unlock wakes the next waiter. Otherwise, the current lock owner
will do so.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: 5ef64cc898 ("mm: allow a controlled amount of unfairness in the page lock")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[ david: clarify and simplify comment + description ]
Reviewed-by: Rik van Riel <riel@surriel.com>
Acked-by: Pedro Falcato <pfalcato@suse.de>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-7-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
This commit is contained in:
Josef Bacik 2026-10-31 10:39:36 +01:00 committed by David Hildenbrand (Arm)
parent dc24e27324
commit 8ccb6ded6a
1 changed files with 15 additions and 3 deletions
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18
mm/filemap.c
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@ -1373,10 +1373,22 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,
* return value based on that state without races.
*
* Also note that WQ_FLAG_WOKEN is sufficient for a non-exclusive
* waiter, but an exclusive one requires WQ_FLAG_DONE.
* waiter, but an exclusive one must also acquire the bit.
*/
if (behavior == EXCLUSIVE)
return wait->flags & WQ_FLAG_DONE ? 0 : -EINTR;
if (behavior == EXCLUSIVE) {
if (wait->flags & WQ_FLAG_DONE)
return 0;
/*
* We might have just been woken up and the waker will not
* wake up anybody else. If we ignore that now, the remaining
* waiters may not get woken up: take the bit now if it is
* still free.
*/
if ((wait->flags & WQ_FLAG_WOKEN) &&
!test_and_set_bit(bit_nr, folio_flags(folio, 0)))
return 0;
return -EINTR;
}
return wait->flags & WQ_FLAG_WOKEN ? 0 : -EINTR;
}