On Fri, Sep 29, 2017 at 01:01:12AM +1000, Nicholas Piggin wrote:

That's fine. If a user is not bound to a subset of CPUs, they could
also cause disturbances with other syscalls and faults, taking locks,
causing tlb flushes and IPIs and things.

So on the big SGI class machines we've had trouble with
for_each_cpu() loops before, and IIRC the biggest Power box is not too
far from that 1-2K CPUs IIRC.

Bouncing that lock across the machine is *painful*, I have vague
memories of cases where the lock ping-pong was most the time spend.

But only Power needs this, all the other architectures are fine with the
lockless approach for MEMBAR_EXPEDITED_PRIVATE.

The ISYNC variant of the same however appears to want TIF flags or
something to aid a number of archs, the rq->lock will not help there.