linux-kernel - Re: Regression: epoll edge-triggered (EPOLLET) for pipes/FIFOs

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Date:   Mon, 12 Oct 2020 23:51:37 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     mtk.manpages@...il.com, Alexander Viro <aviro@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Ian Kent <raven@...maw.net>,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Davide Libenzi <davidel@...ilserver.org>
Subject: Re: Regression: epoll edge-triggered (EPOLLET) for pipes/FIFOs

On 10/12/20 10:52 PM, Linus Torvalds wrote:
> On Mon, Oct 12, 2020 at 1:30 PM Michael Kerrisk (man-pages)
> <mtk.manpages@...il.com> wrote:
>>
>> [CC += Davide]
> 
> I'm not sure how active Davide is any more..

Yep, I know. But just in case.

>> I don't think this is correct. The epoll(7) manual page
>> sill carries the text written long ago by Davide Libenzi,
>> the creator of epoll:
>>
>>     Since  even with edge-triggered epoll, multiple events can be gen‐
>>     erated upon receipt of multiple chunks of data, the caller has the
>>     option  to specify the EPOLLONESHOT flag, to tell epoll to disable
>>     the associated file descriptor after the receipt of an event  with
>>     epoll_wait(2).
>>
>> My reading of that text is that in the scenario that I describe a
>> readiness notification should be generated at step 3 (and indeed
>> should be generated whenever additional data bleeds into the channel).
> 
> Hmm.
> 
> That is unfortunate, because it basically exposes an internal wait
> queue implementation decision, not actual real semantics.

I don't disagree that the longstanding semantics are a little odd;
your comment explains perhaps why.

> I suspect it's easy enough to "fix" the regression with the attached
> patch. It's pretty nonsensical, but I guess there's not a lot of
> downside - if the pipe wasn't empty, there normally shouldn't be any
> non-epoll readers anyway.
> 
> I'm busy merging, mind testing this odd patch out? It is _entirely_
> untested, but from the symptoms I think it's the obvious fix.

Applied against current master (13cb73490f475). My test now
runs as I expected.

> I did the same thing for the "reader starting out from a full pipe" case too.

I haven't tested this, but thanks for thinking of it.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/