[PATCH] tee: link_pipe() must be careful when dropping one of the pipe locks
We need to ensure that we only drop a lock that is ordered last, to avoid
ABBA deadlocks with competing processes.
Signed-off-by: Jens Axboe <axboe@suse.de>
diff --git a/fs/splice.c b/fs/splice.c
index 78cd264..4f5e6b0 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1012,7 +1012,9 @@
size_t len, unsigned int flags)
{
struct pipe_buffer *ibuf, *obuf;
- int ret = 0, do_wakeup = 0, i;
+ int ret, do_wakeup, i, ipipe_first;
+
+ ret = do_wakeup = ipipe_first = 0;
/*
* Potential ABBA deadlock, work around it by ordering lock
@@ -1020,6 +1022,7 @@
* could deadlock (one doing tee from A -> B, the other from B -> A).
*/
if (ipipe->inode < opipe->inode) {
+ ipipe_first = 1;
mutex_lock(&ipipe->inode->i_mutex);
mutex_lock(&opipe->inode->i_mutex);
} else {
@@ -1068,9 +1071,11 @@
/*
* We have input available, but no output room.
- * If we already copied data, return that.
+ * If we already copied data, return that. If we
+ * need to drop the opipe lock, it must be ordered
+ * last to avoid deadlocks.
*/
- if (flags & SPLICE_F_NONBLOCK) {
+ if ((flags & SPLICE_F_NONBLOCK) || !ipipe_first) {
if (!ret)
ret = -EAGAIN;
break;
@@ -1104,7 +1109,12 @@
if (ret)
break;
}
- if (flags & SPLICE_F_NONBLOCK) {
+ /*
+ * pipe_wait() drops the ipipe mutex. To avoid deadlocks
+ * with another process, we can only safely do that if
+ * the ipipe lock is ordered last.
+ */
+ if ((flags & SPLICE_F_NONBLOCK) || ipipe_first) {
if (!ret)
ret = -EAGAIN;
break;