linux-kernel - Re: Why the edge-triggered mode doesn't work for epoll file descriptor?

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Date:   Mon, 19 Aug 2019 08:58:04 +0800
From:   Heiher <r@....cc>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Why the edge-triggered mode doesn't work for epoll file descriptor?

On Sun, Aug 18, 2019 at 12:36 AM Heiher <r@....cc> wrote:
>
> Hello,
>
> I've added a pipe file descriptor (fd1) to an epoll (fd3) with
> EPOLLOUT in edge-triggered mode, and then added the fd3 to another
> epoll (fd4) with EPOLLIN in edge-triggered too.
>
> Next, waiting for fd4 without timeout. When fd1 to be writable, i
> think epoll_wait(fd4, ...)  only return once, because all file
> descriptors are added in edge-triggered mode.
>
> But, the actual result is returns many and many times until do once
> eopll_wait(fd3, ...).
>
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/epoll.h>
>
> int
> main (int argc, char *argv[])
> {
>     int efd[2];
>     struct epoll_event e;
>
>     efd[0] = epoll_create (1);
>     if (efd[0] < 0)
>         return -1;
>
>     efd[1] = epoll_create (1);
>     if (efd[1] < 0)
>         return -2;
>
>     e.events = EPOLLIN | EPOLLET;
>     e.data.u64 = 1;
>     if (epoll_ctl (efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
>         return -3;
>
>     e.events = EPOLLOUT | EPOLLET;
>     e.data.u64 = 2;
>     if (epoll_ctl (efd[1], EPOLL_CTL_ADD, 1, &e) < 0)
>         return -4;
>
>     for (;;) {
>         struct epoll_event events[16];
>         int nfds;
>
>         nfds = epoll_wait (efd[0], events, 16, -1);
>         printf ("nfds: %d\n", nfds);
>     }
>
>     close (efd[1]);
>     close (efd[0]);
>
>     return 0;
> }
>
> --
> Best regards!
> Hev
> https://hev.cc

Is this behavior correct? any help?

-- 
Best regards!
Hev
https://hev.cc