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

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: 5ef64cc8987a ("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>

0 files changed, 0 insertions, 0 deletions