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[209.85.167.52]) by smtp.gmail.com with ESMTPSA id u10sm4345857lja.110.2020.10.12.13.52.32 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 12 Oct 2020 13:52:32 -0700 (PDT) Received: by mail-lf1-f52.google.com with SMTP id 77so9504692lfl.2 for ; Mon, 12 Oct 2020 13:52:32 -0700 (PDT) X-Received: by 2002:a19:4186:: with SMTP id o128mr7972344lfa.148.1602535951534; Mon, 12 Oct 2020 13:52:31 -0700 (PDT) MIME-Version: 1.0 References: <81229415-fb97-51f7-332c-d5e468bcbf2a@gmail.com> In-Reply-To: <81229415-fb97-51f7-332c-d5e468bcbf2a@gmail.com> From: Linus Torvalds Date: Mon, 12 Oct 2020 13:52:15 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: Regression: epoll edge-triggered (EPOLLET) for pipes/FIFOs To: "Michael Kerrisk (man-pages)" Cc: Alexander Viro , David Howells , Rasmus Villemoes , Greg Kroah-Hartman , Peter Zijlstra , Nicolas Dichtel , Ian Kent , Christian Brauner , keyrings@vger.kernel.org, "linux-fsdevel@vger.kernel.org" , Linux API , lkml , Davide Libenzi Content-Type: multipart/mixed; boundary="00000000000021becd05b17f7ab4" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --00000000000021becd05b17f7ab4 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, Oct 12, 2020 at 1:30 PM Michael Kerrisk (man-pages) wrote: > > [CC +=3D Davide] I'm not sure how active Davide is any more.. > 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=E2= =80=90 > 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 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. I did the same thing for the "reader starting out from a full pipe" case to= o. 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