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Date: Wed, 21 Oct 2009 19:28:54 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH] qemu: work around for "posix-aio-compat"
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>, qemu-devel@nongnu.org

> I've uploaded them here:
> http://www.kernel.org/pub/linux/kernel/people/mst/
> you can't see them in mirrors yet but will be able to soon when
> kernel.org mirroring system catches them.

There is no difference in optimizations except that here:

         for (i = 0; i < aiocb->aio_niov && count; ++i) {

one of the two versions actually does "count && i < aiocb->aio_niov" due 
to hashing vagaries.  This is irrelevant anyway.  Same inlining, same 
loop optimization decisions, same everything else.  So a GCC bug can be 
ruled out, IMHO.

The only difference, as someone already suspected, is the padding---the 
sigset is placed between the top of the frame and the other variables, 
which may hide an overrun.  This is quite amazing for a function that 
has no arrays, but still is the only evidence.

I suggest trying to make the sigset_t static, since that generates 
exactly the same code as the "nohang" case, and exactly the same stack 
layout as the "hang" case.  The next obvious step would be placing a 
watchpoint somewhere.

Cheers,

Paolo