linux-kernel - Re: [PATCH AUTOSEL 5.13 001/104] pipe: make pipe writes always wake up readers

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Message-ID: <YQfPZAytBFdFJeH7@kroah.com>
Date:   Mon, 2 Aug 2021 12:56:36 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sandeep Patil <sspatil@...roid.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.13 001/104] pipe: make pipe writes always wake
 up readers

On Mon, Aug 02, 2021 at 06:46:48AM -0400, Sasha Levin wrote:
> From: Linus Torvalds <torvalds@...ux-foundation.org>
> 
> 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@mail.gmail.com/ [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@mail.gmail.com/ [4]
> Link: https://lore.kernel.org/lkml/20210729222635.2937453-1-sspatil@android.com/
> Reported-by: Sandeep Patil <sspatil@...roid.com>
> Cc: Michael Kerrisk <mtk.manpages@...il.com>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  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