Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s

[Jan Kiszka <jan.kiszka@web.de>]

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M. Warner Losh wrote:
> In message: <20081104113204.GA32125@shareable.org>
>             Jamie Lokier <jamie@shareable.org> writes:
> : andrzej zaborowski wrote:
> : > > My man page even warns that the Linux
> : > > kernel is not implementing it yet, though I don't think this still
> : > > applies to recent 2.6.2x kernels.
> : > 
> : > According to the man page it moved to kernel at 2.6.16 but the glibc
> : > wrapper should be ok too.
> : 
> : If there's a glibc wrapper, it cannot be reliable...
> : 
> : *Looks at glibc source*
> : 
> : That's right.  The glibc pselect() wrapper has the same race condition
> : which prompted this QEMU bug.  If the signal arrives after unmasking
> : and before select() in the wrapper, then blocks.
> : 
> : In other words, don't use pselect() if you might run on a kernel older
> : than 2.6.16, or on a host architecture which adds pselect() in a later
> : kernel version.  Also, I wouldn't be surprised if older versions of
> : some BSDs have similar dodgy wrappers.
> 
> Which ones have a good kernel implementation of it?  FreeBSD's is
> currently approximately:
> 
> 	if (!mask)
> 		_sigprocmask(mask, &oldmask);
> 	/* here */
> 	select();
> 	if (!mask)
> 		_sigprocmask(oldmask, NULL);
> 
> I'm assuming that the problem is due to a signal arriving at /* here */.

I guess those things happen under some kind of preemption lock,
otherwise it would be a really poor implementation.

However, think we buried the idea of using pselect for this anyway.

Jan


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