On Mon, Aug 02, 2021 at 06:46:48AM -0400, Sasha Levin wrote:
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>
> commit 3a34b13a88caeb2800ab44a4918f230041b37dd9 upstream.
>
> Since commit 1b6b26ae7053 ("pipe: fix and clarify pipe write wakeup
> logic") we have sanitized the pipe write logic, and would only try to
> wake up readers if they needed it.
>
> In particular, if the pipe already had data in it before the write,
> there was no point in trying to wake up a reader, since any existing
> readers must have been aware of the pre-existing data already. Doing
> extraneous wakeups will only cause potential thundering herd problems.
>
> However, it turns out that some Android libraries have misused the EPOLL
> interface, and expected "edge triggered" be to "any new write will
> trigger it". Even if there was no edge in sight.
>
> Quoting Sandeep Patil:
> "The commit 1b6b26ae7053 ('pipe: fix and clarify pipe write wakeup
> logic') changed pipe write logic to wakeup readers only if the pipe
> was empty at the time of write. However, there are libraries that
> relied upon the older behavior for notification scheme similar to
> what's described in [1]
>
> One such library 'realm-core'[2] is used by numerous Android
> applications. The library uses a similar notification mechanism as GNU
> Make but it never drains the pipe until it is full. When Android moved
> to v5.10 kernel, all applications using this library stopped working.
>
> The library has since been fixed[3] but it will be a while before all
> applications incorporate the updated library"
>
> Our regression rule for the kernel is that if applications break from
> new behavior, it's a regression, even if it was because the application
> did something patently wrong. Also note the original report [4] by
> Michal Kerrisk about a test for this epoll behavior - but at that point
> we didn't know of any actual broken use case.
>
> So add the extraneous wakeup, to approximate the old behavior.
>
> [ I say "approximate", because the exact old behavior was to do a wakeup
> not for each write(), but for each pipe buffer chunk that was filled
> in. The behavior introduced by this change is not that - this is just
> "every write will cause a wakeup, whether necessary or not", which
> seems to be sufficient for the broken library use. ]
>
> It's worth noting that this adds the extraneous wakeup only for the
> write side, while the read side still considers the "edge" to be purely
> about reading enough from the pipe to allow further writes.
>
> See commit f467a6a66419 ("pipe: fix and clarify pipe read wakeup logic")
> for the pipe read case, which remains that "only wake up if the pipe was
> full, and we read something from it".
>
> Link: https://lore.kernel.org/lkml/CAHk-=wjeG0q1vgzu4iJhW5juPkTsjTYmiqiMUYAebWW+0bam6w@xxxxxxxxxxxxxx/ [1]
> Link: https://github.com/realm/realm-core [2]
> Link: https://github.com/realm/realm-core/issues/4666 [3]
> Link: https://lore.kernel.org/lkml/CAKgNAkjMBGeAwF=2MKK758BhxvW58wYTgYKB2V-gY1PwXxrH+Q@xxxxxxxxxxxxxx/ [4]
> Link: https://lore.kernel.org/lkml/20210729222635.2937453-1-sspatil@xxxxxxxxxxx/
> Reported-by: Sandeep Patil <sspatil@xxxxxxxxxxx>
> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
> fs/pipe.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
This is already in the 5.13 queue, did you mean to send this again?
thanks,
greg k-h
