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Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s
From: |
Daniel P. Berrange |
Subject: |
Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s |
Date: |
Wed, 5 Nov 2008 20:28:52 +0000 |
User-agent: |
Mutt/1.4.1i |
On Wed, Nov 05, 2008 at 09:16:59PM +0100, andrzej zaborowski wrote:
> 2008/11/5 Jan Kiszka <address@hidden>:
> > M. Warner Losh wrote:
> >> In message: <address@hidden>
> >> Jamie Lokier <address@hidden> writes:
> >> : M. Warner Losh wrote:
> >> : > : 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
> >> */.
> >> :
> >> : If that's _kernel_ code and the kernel behaves like Linux, it's not a
> >> : problem because signals don't affect the control flow until returning
> >> : to userspace, meaning the select() will return EINTR.
> >>
> >> It is currently user level code, and I'm looking at moving it into the
> >> kernel, but I need to understand the race being talked about here.
> >
> > From the Linux man page on [p]select:
> >
> > "The reason that pselect() is needed is that if one wants to wait for
> > either a signal or for a file descriptor to become ready, then an atomic
> > test is needed to prevent race conditions. (Suppose the signal handler
> > sets a global flag and returns. Then a test of this global flag followed
> > by a call of select() could hang indefinitely if the signal arrived just
> > after the test but just before the call. By contrast, pselect() allows
> > one to first block signals, handle the signals that have come in, then
> > call pselect() with the desired sigmask, avoiding the race.)"
> >
> > So the unmasking and possible blocking on select must be done
> > atomically. And that is only feasible in kernel land.
>
> To be exact, it *was* possible for glibc to implement a pselect free of races:
> that is by using the same trick as your patch, i.e. making a pipe and
> adding it to select()ed fd's and mangling the sigmask.
Yes & no. The trouble with glibc using pipes behind your back is that
then it creates a totally different race in threaded apps, where a FD
could be leaked to a child process between glibc opening its secret
pipe and setting the O_CLOSEXEC flag. Indeed it already suffers from
this problem with name resolving
Daniel
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- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, (continued)
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Jamie Lokier, 2008/11/04
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, M. Warner Losh, 2008/11/04
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Jan Kiszka, 2008/11/04
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, M. Warner Losh, 2008/11/04
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Jan Kiszka, 2008/11/04
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Jamie Lokier, 2008/11/05
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, M. Warner Losh, 2008/11/05
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Jan Kiszka, 2008/11/05
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Daniel P. Berrange, 2008/11/05
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, andrzej zaborowski, 2008/11/05
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s,
Daniel P. Berrange <=
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Jamie Lokier, 2008/11/05
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Jamie Lokier, 2008/11/05
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, M. Warner Losh, 2008/11/06
- Re: [Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s, Avi Kivity, 2008/11/04