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Subject[PATCH 4.14 052/115] fuse: fix possibly missed wake-up after abort
Date
[ Upstream commit 2d84a2d19b6150c6dbac1e6ebad9c82e4c123772 ]

In current fuse_drop_waiting() implementation it's possible that
fuse_wait_aborted() will not be woken up in the unlikely case that
fuse_abort_conn() + fuse_wait_aborted() runs in between checking
fc->connected and calling atomic_dec(&fc->num_waiting).

Do the atomic_dec_and_test() unconditionally, which also provides the
necessary barrier against reordering with the fc->connected check.

The explicit smp_mb() in fuse_wait_aborted() is not actually needed, since
the spin_unlock() in fuse_abort_conn() provides the necessary RELEASE
barrier after resetting fc->connected. However, this is not a performance
sensitive path, and adding the explicit barrier makes it easier to
document.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: b8f95e5d13f5 ("fuse: umount should wait for all requests")
Cc: <stable@vger.kernel.org> #v4.19
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
fs/fuse/dev.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 770733106d6d4..c934fab444529 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -133,9 +133,13 @@ static bool fuse_block_alloc(struct fuse_conn *fc, bool for_background)

static void fuse_drop_waiting(struct fuse_conn *fc)
{
- if (fc->connected) {
- atomic_dec(&fc->num_waiting);
- } else if (atomic_dec_and_test(&fc->num_waiting)) {
+ /*
+ * lockess check of fc->connected is okay, because atomic_dec_and_test()
+ * provides a memory barrier mached with the one in fuse_wait_aborted()
+ * to ensure no wake-up is missed.
+ */
+ if (atomic_dec_and_test(&fc->num_waiting) &&
+ !READ_ONCE(fc->connected)) {
/* wake up aborters */
wake_up_all(&fc->blocked_waitq);
}
@@ -2170,6 +2174,8 @@ EXPORT_SYMBOL_GPL(fuse_abort_conn);

void fuse_wait_aborted(struct fuse_conn *fc)
{
+ /* matches implicit memory barrier in fuse_drop_waiting() */
+ smp_mb();
wait_event(fc->blocked_waitq, atomic_read(&fc->num_waiting) == 0);
}

--
2.20.1


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