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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 f467a6a6 and 1b6b26ae ("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 3a34b13a ("pipe: make pipe
writes always wake up readers") only to be broken again when by commit
3b844826 ("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: 3b844826

 ("pipe: avoid unnecessary EPOLLET wakeups under normal loads")
Reported-by: kernel test robot <oliver.sang@intel.com>
Tested-by: Oliver Sang <oliver.sang@intel.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>