Commit 2d84a2d1 authored Nov 09, 2018 by Miklos Szeredi

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: b8f95e5d ("fuse: umount should wait for all requests")
Cc: <stable@vger.kernel.org> #v4.19

parent 7fabaf30

fs/fuse/dev.c

+9 −3

Original line number	Diff line number	Diff line
		@@ -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);
		}

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