RE: [Xen-devel] xenbus stress testing

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

James Harper writes ("RE: [Xen-devel] xenbus stress testing"):
> I think I have found the error and it was probably a 1 in a million race
> so stress testing might not have helped anyway. My code went:
> 
> len = min(ring->rsp_prod - ring->rsp_cons, msg_size)
> 
> and the ASSERT was hit because len was > msg_size, and the only possible
> way I can ever see that happening is if ring->rsp_prod changed between
> the if in the min() and the assignment. I'm now snapshotting rsp_prod to
> a local variable at the start. Kind of embarrassing really as plenty of
> example code exists.

You need to think about memory barriers and/or volatile.  Simply
"snapshotting" with an ordinary assignment doesn't work.

I don't know how this is done in Windows but the Linux kernel has a
clear explanation of the problem and how it's solved in Linux.  Look
in the kernel source tree in Documentation/memory-barriers.txt.

Ian.

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