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Subject[4.19-stable PATCH 2/2] dax: Use non-exclusive wait in wait_entry_unlocked()
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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@infradead.org>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.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)
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