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

[M. Warner Losh]

In message: <address@hidden>
            Jamie Lokier <address@hidden> 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 */.

Warner