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Subject: Re: [PATCH 1/1] fs: pipe: wakeup readers everytime new data written
 is to pipe
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable <stable@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>
References: <20210729222635.2937453-1-sspatil@android.com>
 <20210729222635.2937453-2-sspatil@android.com>
 <CAHk-=wh-DWvsFykwAy6uwyv24nasJ39d7SHT+15x+xEXBtSm_Q@mail.gmail.com>
 <cee514d6-8551-8838-6d61-098d04e226ca@android.com>
 <CAHk-=wjStQurUzSAPVajL6Rj=CaPuSSgwaMO=0FJzFvSD66ACw@mail.gmail.com>
From:   Sandeep Patil <sspatil@android.com>
Message-ID: <b1688f32-cb0e-04e1-3c91-aa8cddbcf41d@android.com>
Date:   Fri, 30 Jul 2021 19:47:22 +0000
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On 7/30/21 7:23 PM, Linus Torvalds wrote:
> On Fri, Jul 30, 2021 at 12:11 PM Sandeep Patil <sspatil@android.com> wrote:
>>
>> Yes, your patch fixes all apps on Android I can test that include this
>> library.
> 
> Ok, thanks for checking.
> 
>> fwiw, the library seems to have been fixed. However, I am not sure
>> how long it will be for all apps to take that update :(.
> 
> I wonder if I could make the wakeup logic do this only for the epollet case.

aren't we supposed to wakeup on each write in level-triggered (default) 
case though?

> 
> I'll have to think about it, but maybe I'll just apply that simple
> patch. I dislike the pointless wakeups, and as long as the only case I
> knew of was only a test of broken behavior, it was fine. But now that
> you've reported actual application breakage, this is in the "real
> regression" category, and so I'll fix it one way or the other.
> 
> And on the other hand I also have a slight preference towards your
> patch simply because you did the work of finding this out, so you
> should get the credit.

Ha, I can't really claim credit here. This was also reported to us
in Android that triggered the search. Plus, now that I see your thread 
with Michael Kerrisk, he was way ahead of us in finding this out.

> 
> I'll mull it over a bit more, but whatever I'll do I'll do before rc4
> and mark it for stable.

Thanks, I was actually going to suggest taking your patch cause it also 
  makes changes in pipe_read(). I am not sure if there are apps that do 
EPOLLET | EPOLLOUT (can't think of a reason why).

- ssp

> 
> Thanks for testing,
> 
>                   Linus
>