| Message ID | 65b22cd4e8bb142c5b7b86bc33fb08de6f318089.1577017472.git.jstancek@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | pipe: fix empty pipe check in pipe_write() | expand |
On Sun, Dec 22, 2019 at 4:35 AM Jan Stancek <jstancek@redhat.com> wrote: > Problem is that after pipe_write() reacquires pipe lock, it > re-checks for empty pipe with potentially stale 'head' and > doesn't wake up read side anymore. pipe->tail can advance > beyond 'head', because there are multiple writers. Thank you. Patch is obviously correct, applied. I wonder how much that whole "cache head/tail/mask" really helps, and if we should strive to get rid of it entirely (and just make "pipe_emptuy()" and friends take a 'const struct pipe_inode_info *" argument). Oh well. I've apple your one-liner, but next time I might decide the cleverness and slight code generation advantage might not be worth it. Hopefully there won't _be_ a next time, of course ;) Linus
diff --git a/fs/pipe.c b/fs/pipe.c index 04d004ee2e8c..57502c3c0fba 100644 --- a/fs/pipe.c +++ b/fs/pipe.c @@ -581,7 +581,7 @@ static inline bool pipe_writable(const struct pipe_inode_info *pipe) } wait_event_interruptible(pipe->wait, pipe_writable(pipe)); __pipe_lock(pipe); - was_empty = pipe_empty(head, pipe->tail); + was_empty = pipe_empty(pipe->head, pipe->tail); } out: __pipe_unlock(pipe);
LTP pipeio_1 test is hanging with v5.5-rc2-385-gb8e382a185eb, with read side observing empty pipe and sleeping and write side running out of space and then sleeping as well. In this scenario there are 5 writers and 1 reader. Problem is that after pipe_write() reacquires pipe lock, it re-checks for empty pipe with potentially stale 'head' and doesn't wake up read side anymore. pipe->tail can advance beyond 'head', because there are multiple writers. Use pipe->head for empty pipe check after reacquiring lock to observe current state. Testing: With patch, LTP pipeio_1 ran successfully in loop for 1 hour. Without patch it hanged within a minute. Fixes: 1b6b26ae7053 ("pipe: fix and clarify pipe write wakeup logic") Reported-by: Rachel Sibley <rasibley@redhat.com> Signed-off-by: Jan Stancek <jstancek@redhat.com> --- fs/pipe.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)