[openib-general] [RFC/BUG] DMA vs. CQ race

[Roland Dreier]

 > A first-cut at a patch was sent out, some very reasonable
 > objections were raised, and the thread fizzled out.

Sorry, I meant to respond again, but I never got around to it.

 > The biggest concern with the earlier patch seemed to be
 > backward compatibility. There was a stab at addressing
 > that in http://tinyurl.com/2x3s52, but no commentary.
 > (Too ugly for words?)

I think you went off into the weeds there, but I'll respond to that
earlier email in detail.

 > Any suggestions as to how to proceed? Should I just code
 > something up in order to have a concrete target to discuss?
 > Or are there any new thoughts based on the previous emails?

I actually have a vague plan for a somewhat cleaner way to get this
fix.  For a variety of reasons, I am planning on changing the way the
kernel handles memory registration so that low-level drivers have more
control over what happens.  This would allow us to folow Gleb's
suggestion to use register MR to create and map the kernel's buffer
and avoid some of the error path ugliness.  So I would prefer to map
the coherent memory that way.

However this will take a while to come to fruition, since it is kind
of a background task for me.  How severe is this issue?  In other
words, when you produced the problem, was it a synthetic test, or a
workload that someone might actually want to run?

 - R.