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

From: Jamie Lokier
Subject: Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s
Date: Tue, 4 Nov 2008 11:32:04 +0000
User-agent: Mutt/1.5.13 (2006-08-11)

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.

-- Jamie

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