From: Miklos Szeredi mszeredi@redhat.com
commit 2d84a2d19b6150c6dbac1e6ebad9c82e4c123772 upstream.
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 Cc: Guenter Roeck linux@roeck-us.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/fuse/dev.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)
--- a/fs/fuse/dev.c +++ b/fs/fuse/dev.c @@ -132,9 +132,13 @@ static bool fuse_block_alloc(struct fuse
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); } @@ -2164,6 +2168,8 @@ 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); }