linux-kernel - Re: F_DUPFD_CLOEXEC implementation

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Message-ID: <Pine.LNX.4.64.0709301709490.19355@alien.or.mcafeemobile.com>
Date:	Sun, 30 Sep 2007 20:15:10 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Denys Vlasenko <vda.linux@...glemail.com>
cc:	Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: F_DUPFD_CLOEXEC implementation

On Mon, 1 Oct 2007, Denys Vlasenko wrote:

> My use case is: I want to do a nonblocking read on descriptor 0 (stdin).
> It may be a pipe or a socket.
> 
> There may be other processes which share this descriptor with me,
> I simply cannot know that. And they, too, may want to do reads on it.
> 
> I want to do nonblocking read in such a way that neither those other
> processes will ever see fd switching to O_NONBLOCK and back, and
> I also want to be safe from other processes doing the same.
> 
> I don't see how this can be done using standard unix primitives.

Indeed. You could simulate non-blocking using poll with zero timeout, but 
if another task may read/write on it, your following read/write may end up 
blocking even after a poll returned the required events.
One way to solve this would be some sort of readx/writex where you pass an 
extra flags parameter (this could be done with sys_indirect, assuming 
we'll ever get that mainline) where you specify the non-blocking 
requirement for-this-call, and not as global per-file flag. Then, of 
course, you'll have to modify all the "file->f_flags & O_NONBLOCK" tests 
(and there are many of them) to check for that flag too (that can be a 
per task_struct flag).

- Davide

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