[openib-general] Race in mthca_cmd_post()

[Roland Dreier]

 > I just took a quick look at asm-ia64/io.h and there is __ia64_mf_a
 > barriers after all non-posted IO operations (ib/outb). config write and
 > config read transcations have identical rules to IO transactions at
 > the PCI bus level.
 > 
 > I'm going to go out on a limb here and say that if Linux code assumes
 > strong ordering of IO operations then it makes sense to also assume
 > strong ordering on config writes. So, instead of patching mthca with
 > this barrier it should go in the Altix config access mechanism..

I don't really know what mf.a does on ia64, but it seems likely that
even if the CPU issues and retires the reads and writes in order
(which is all a CPU barrier is likely to do), we would still have a
problem where the PCI-X host bridge allows the MMIO read to pass the
config write, because the config write is still pending a split
completion on the bus.

And a quick web search finds "interesting" stuff like this:

 > Platform-acceptance is a tricky business, as it's, well, platform
 > dependent (note that "mf.a" doesn't really guarantee to do anything).

So I'm inclined to think this patch is correct.  However, I'll check
with linux-pci and linux-ia64 before asking Linus to merge it.

Thanks,
  Roland