Subject: Re: Thread safety for epoll/libaio
From: Anton Titov <a.titov@host.bg>
To: "Li, Peng" <ringer9cs@gmail.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
In-Reply-To: <598a055d0602281236m7eac9c09oc60af9ce28e7e4bf@mail.gmail.com>
References: <598a055d0602281236m7eac9c09oc60af9ce28e7e4bf@mail.gmail.com>
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Organization: Host.bg
Date: Wed, 01 Mar 2006 03:55:42 +0200
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On Tue, 2006-02-28 at 15:36 -0500, Li, Peng wrote:

> Thread B:  while(1) { epoll_wait();  ... }
> // same as thread A
> Thread D:  ... epoll_ctl(); ....
> 
> Suppose thread B calls epoll_wait and blocks before thread D calls
> epoll_ctl.  Is it safe to do so? Will thread B be notified for the
> event submitted by thread D? 

Hello,

I have some (more) expirience with epoll and threads and it seem to work
well. If you have epoll_wait() in one thread and another thread do
epoll_ctl to add a handle, epoll_wait will wake up as soon as the handle
is ready for operation (most of the time instantly, when operation is
write). 

epoll man page states:

              Q6     Will the close of an fd cause it to be removed from
all epoll sets automatically?

              A6     Yes.
but I was experiencing some (rare) segfaults with my application while
benchmarking it when I just closing my descriptors. Debugging showed,
that I'm getting events for destroyed objects (with closed descriptors).
Adding epoll_ctl(..., EPOLL_CTL_DEL, ...) fixed this.


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