[PATCH 07/46] mm/filemap: don't lose a folio lock wakeup to a signal

["David Hildenbrand (Arm)" <david@kernel.org>]

From: Josef Bacik <josef@toxicpanda.com>

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 ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/filemap.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 906b013c11eb3..6b5e1d866a9e0 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -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;
 }

-- 
2.43.0