[ofa-general] Re: [PATCH 24/28] IB/ipath - ipath_poll fixups and enhancements

[Roland Dreier]

 > > OK, fair enough, although it seems you may be missing some memory
 > > barriers to make sure you don't run into the CPU reordering accesses
 > > to the head/tail pointers.
 > 
 > i had a quick look at the patch and the surrounding
 > code and i did not catch the problem.  can you be a
 > little more specific about the suspect code?

I'm not sure there's a bug there.  But the patch in question does

 > +       tail = *(volatile u64 *)pd->port_rcvhdrtail_kvaddr;

with no memory ordering.  The volatile makes sure the compiler puts
that read where you wrote it, but there's no guarantee that the CPU
executes it anywhere remotely close to where it is in the code.  Later
on you have

 > +       if (tail != head ||
 > +           test_bit(IPATH_PORT_WAITING_RCV, &pd->int_flag)) {

etc., and the CPU might speculate those test far ahead of actually
reading the port_rcvhdrttail_kvaddr value, which means you might end
up executing code based on a guess about tail != head that is not true
at the time it speculates the branch, but by the time it does get to
actually check its speculation, the guess has become true.

Just something to think about...