From fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Tue, 24 Aug 2021 10:39:25 -0700 Subject: pipe: do FASYNC notifications for every pipe IO, not just state changes It turns out that the SIGIO/FASYNC situation is almost exactly the same as the EPOLLET case was: user space really wants to be notified after every operation. Now, in a perfect world it should be sufficient to only notify user space on "state transitions" when the IO state changes (ie when a pipe goes from unreadable to readable, or from unwritable to writable). User space should then do as much as possible - fully emptying the buffer or what not - and we'll notify it again the next time the state changes. But as with EPOLLET, we have at least one case (stress-ng) where the kernel sent SIGIO due to the pipe being marked for asynchronous notification, but the user space signal handler then didn't actually necessarily read it all before returning (it read more than what was written, but since there could be multiple writes, it could leave data pending). The user space code then expected to get another SIGIO for subsequent writes - even though the pipe had been readable the whole time - and would only then read more. This is arguably a user space bug - and Colin King already fixed the stress-ng code in question - but the kernel regression rules are clear: it doesn't matter if kernel people think that user space did something silly and wrong. What matters is that it used to work. So if user space depends on specific historical kernel behavior, it's a regression when that behavior changes. It's on us: we were silly to have that non-optimal historical behavior, and our old kernel behavior was what user space was tested against. Because of how the FASYNC notification was tied to wakeup behavior, this was first broken by commits f467a6a66419 and 1b6b26ae7053 ("pipe: fix and clarify pipe read/write wakeup logic"), but at the time it seems nobody noticed. Probably because the stress-ng problem case ends up being timing-dependent too. It was then unwittingly fixed by commit 3a34b13a88ca ("pipe: make pipe writes always wake up readers") only to be broken again when by commit 3b844826b6c6 ("pipe: avoid unnecessary EPOLLET wakeups under normal loads"). And at that point the kernel test robot noticed the performance refression in the stress-ng.sigio.ops_per_sec case. So the "Fixes" tag below is somewhat ad hoc, but it matches when the issue was noticed. Fix it for good (knock wood) by simply making the kill_fasync() case separate from the wakeup case. FASYNC is quite rare, and we clearly shouldn't even try to use the "avoid unnecessary wakeups" logic for it. Link: https://lore.kernel.org/lkml/20210824151337.GC27667@xsang-OptiPlex-9020/ Fixes: 3b844826b6c6 ("pipe: avoid unnecessary EPOLLET wakeups under normal loads") Reported-by: kernel test robot Tested-by: Oliver Sang Cc: Eric Biederman Cc: Colin Ian King Signed-off-by: Linus Torvalds --- fs/pipe.c | 20 ++++++++------------ 1 file changed, 8 insertions(+), 12 deletions(-) (limited to 'fs/pipe.c') diff --git a/fs/pipe.c b/fs/pipe.c index 678dee2a8228..6d4342bad9f1 100644 --- a/fs/pipe.c +++ b/fs/pipe.c @@ -363,10 +363,9 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to) * _very_ unlikely case that the pipe was full, but we got * no data. */ - if (unlikely(was_full)) { + if (unlikely(was_full)) wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM); - kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT); - } + kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT); /* * But because we didn't read anything, at this point we can @@ -385,12 +384,11 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to) wake_next_reader = false; __pipe_unlock(pipe); - if (was_full) { + if (was_full) wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM); - kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT); - } if (wake_next_reader) wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM); + kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT); if (ret > 0) file_accessed(filp); return ret; @@ -565,10 +563,9 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from) * become empty while we dropped the lock. */ __pipe_unlock(pipe); - if (was_empty) { + if (was_empty) wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM); - kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); - } + kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); wait_event_interruptible_exclusive(pipe->wr_wait, pipe_writable(pipe)); __pipe_lock(pipe); was_empty = pipe_empty(pipe->head, pipe->tail); @@ -591,10 +588,9 @@ out: * Epoll nonsensically wants a wakeup whether the pipe * was already empty or not. */ - if (was_empty || pipe->poll_usage) { + if (was_empty || pipe->poll_usage) wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM); - kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); - } + kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); if (wake_next_writer) wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM); if (ret > 0 && sb_start_write_trylock(file_inode(filp)->i_sb)) { -- cgit v1.2.3