linux-kernel - Re: epoll design problems with common fork/exec patterns

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Message-ID: <Pine.LNX.4.64.0710270950170.12127@alien.or.mcafeemobile.com>
Date:	Sat, 27 Oct 2007 09:59:07 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Marc Lehmann <linux-kernel@....eu>
cc:	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: epoll design problems with common fork/exec patterns

On Sat, 27 Oct 2007, Marc Lehmann wrote:

> > Please provide some code to illustrate one exact problem you have.
> 
>    // assume there is an open epoll set that listens for events on fd 5
>    if (fork () = 0)
>      {
>        close (5);
>        // fd 5 is now removed from the epoll set of the parent.
>        _exit (0);
>      }

Hmmm ... what? I assume you know that:

1) A file descriptor is a userspace view/handle of a kernel object

2) The kernel object has a use-count for as many file descriptors that 
   have been handed out to userspace

3) A close() decreases the internal counter by one

4) The kernel object gets effectively closed when the internal counter 
   goes to zero

5) A fork() acts as a dup() on the file descriptors by hence bumping up 
   its internal counter

6) Epoll removes the file from the set, when the *kernel* object gets 
   closed (internal use-count goes to zero)

With that in mind, how can the code snippet above trigger a removal from 
the epoll set?



- Davide


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