linux-kernel - Strange EPOLLOUT|EPOLLET behaviour (spurious events)

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Date:	Sun, 22 Apr 2012 17:32:07 +0200
From:	Christof Meerwald <cmeerw@...erw.org>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Strange EPOLLOUT|EPOLLET behaviour (spurious events)

Hi,

I am seeing some unexpected behaviour with EPOLLOUT in edge-triggered
mode - some example code is available from
http://svn.cmeerw.net/src/nginetd/trunk/test/eptest-out.c

What seems to be happening is that I get an EPOLLOUT event after each
send on the socket (even in edge-triggered mode). This is what strace
shows me:

epoll_create(1024)                      = 3
socketpair(PF_FILE, SOCK_DGRAM, 0, [4, 5]) = 0
fcntl(5, F_GETFL)                       = 0x2 (flags O_RDWR)
fcntl(5, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
epoll_ctl(3, EPOLL_CTL_ADD, 5, {EPOLLIN|EPOLLOUT|EPOLLET, {u32=5, u64=5}}) = 0
sendto(5, "\0\0\0\0", 4, 0, NULL, 0)    = 4
recvfrom(4, "\0\0\0\0", 4, 0, NULL, NULL) = 4
epoll_wait(3, {{EPOLLOUT, {u32=5, u64=5}}}, 16, 0) = 1
sendto(5, "\1\0\0\0", 4, 0, NULL, 0)    = 4
recvfrom(4, "\1\0\0\0", 4, 0, NULL, NULL) = 4
epoll_wait(3, {{EPOLLOUT, {u32=5, u64=5}}}, 16, 0) = 1
sendto(5, "\2\0\0\0", 4, 0, NULL, 0)    = 4
recvfrom(4, "\2\0\0\0", 4, 0, NULL, NULL) = 4
epoll_wait(3, {{EPOLLOUT, {u32=5, u64=5}}}, 16, 0) = 1

But as I am using EPOLLET, I wouldn't expect to get any EPOLLOUT
events on that socket as send never returns EAGAIN (and there isn't
any state transition).

BTW, I have seen https://lkml.org/lkml/2011/11/17/48 , but my case is
different (as there isn't any EPOLLIN event on that socket either).

Am I missing something here?


Christof

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