linux-kernel - [4.19-stable PATCH 2/2] dax: Use non-exclusive wait in wait_entry

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Date:   Sat, 05 Jan 2019 11:45:13 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     stable@...r.kernel.org
Cc:     Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, sashal@...nel.org,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: [4.19-stable PATCH 2/2] dax: Use non-exclusive wait in
 wait_entry_unlocked()

commit d8a706414af4827fc0b4b1c0c631c607351938b9 upstream.

get_unlocked_entry() uses an exclusive wait because it is guaranteed to
eventually obtain the lock and follow on with an unlock+wakeup cycle.
The wait_entry_unlocked() path does not have the same guarantee. Rather
than open-code an extra wakeup, just switch to a non-exclusive wait.

Cc: Matthew Wilcox <willy@...radead.org>
Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Reviewed-by: Jan Kara <jack@...e.cz>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---
 fs/dax.c |   16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 415605fafaeb..09fa70683c41 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -275,18 +275,16 @@ static void wait_entry_unlocked(struct address_space *mapping, pgoff_t index,
 	ewait.wait.func = wake_exceptional_entry_func;
 
 	wq = dax_entry_waitqueue(mapping, index, entry, &ewait.key);
-	prepare_to_wait_exclusive(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
+	/*
+	 * Unlike get_unlocked_entry() there is no guarantee that this
+	 * path ever successfully retrieves an unlocked entry before an
+	 * inode dies. Perform a non-exclusive wait in case this path
+	 * never successfully performs its own wake up.
+	 */
+	prepare_to_wait(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
 	xa_unlock_irq(&mapping->i_pages);
 	schedule();
 	finish_wait(wq, &ewait.wait);
-
-	/*
-	 * Entry lock waits are exclusive. Wake up the next waiter since
-	 * we aren't sure we will acquire the entry lock and thus wake
-	 * the next waiter up on unlock.
-	 */
-	if (waitqueue_active(wq))
-		__wake_up(wq, TASK_NORMAL, 1, &ewait.key);
 }
 
 static void unlock_mapping_entry(struct address_space *mapping, pgoff_t index)