linux-kernel - Re: [PATCH 1/1] eventfd: implementation of EFD

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Date:	Sat, 09 Feb 2013 04:26:47 +0100
From:	Martin Sustrik <sustrik@...bpm.com>
To:	Eric Wong <normalperson@...t.net>
CC:	Andy Lutomirski <luto@...capital.net>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sha Zhengju <handai.szj@...bao.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] eventfd: implementation of EFD_MASK flag

On 08/02/13 23:08, Eric Wong wrote:

>>>>> poll(2) function (POLLIN, POLLOUT, POLLERR, POLLHUP etc.) Specified
>>>>> events will
>>>>> be signaled when polling (select, poll, epoll) on the eventfd is done
>>>>> later on.
>>>>> 'ptr' is an opaque pointer that is not interpreted by eventfd object.
>>>>
>>>> How does this interact with EPOLLET?
>>>
>>> That's an interesting question. The original eventfd code doesn't do
>>> anything specific to either edge or level mode. Neither does my patch.
>>>
>>> Inspection of the code seems to suggest that edge vs. level distinction is
>>> handled elsewhere (ep_send_events_proc) where there is a separate list of
>>> ready events and the function, after returning the event, decides whether to
>>> leave the event in the list (level) or delete it from the list (edge).
>
> Right, the edge vs. level distinction is internal to epoll.

I wrote a test program for EFD_MASK+EPOLLET and it seems to behave in 
intuitive kind of way:

int main ()
{
     int fd;
     struct efd_mask mask;
     ssize_t nbytes;
     int rc;
     int ep;
     struct epoll_event epe;

     fd = eventfd (0, EFD_MASK);

     ep = epoll_create (10);
     assert (ep != -1);
     epe.events = EPOLLIN | EPOLLET;
     rc = epoll_ctl (ep, EPOLL_CTL_ADD, fd, &epe);
     assert (rc != -1);

     mask.events = 0;
     nbytes = write (fd, &mask, sizeof (mask));
     assert (nbytes == sizeof (mask));
     rc = epoll_wait (ep, &epe, 1, 100);
     assert (rc == 0);

     mask.events = POLLIN;
     nbytes = write (fd, &mask, sizeof (mask));
     assert (nbytes == sizeof (mask));
     rc = epoll_wait (ep, &epe, 1, 100);
     assert (rc == 1 && epe.events == EPOLLIN);
     rc = epoll_wait (ep, &epe, 1, 100);
     assert (rc == 0);

     mask.events = POLLIN;
     nbytes = write (fd, &mask, sizeof (mask));
     mask.events = 0;
     nbytes = write (fd, &mask, sizeof (mask));
     rc = epoll_wait (ep, &epe, 1, 100);
     assert (rc == 0);

     rc = close (ep);
     assert (rc == 0);
     rc = close (fd);
     assert (rc == 0);

     return 0;
}

Martin
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