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fuse: fix possibly missed wake-up after abort 

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: b8f95e5d13 ("fuse: umount should wait for all requests")
Cc: <stable@vger.kernel.org> #v4.19
This commit is contained in:
Miklos Szeredi 2018-11-09 15:52:16 +01:00
parent 7fabaf3034
commit 2d84a2d19b
1 changed files with 9 additions and 3 deletions
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12
fs/fuse/dev.c
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@ -165,9 +165,13 @@ 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);
}
@ -2221,6 +2225,8 @@ 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);
}