On Wed, Jan 16, 2019 at 5:24 AM Peter Zijlstra [off-list ref] wrote:

On Mon, Jan 14, 2019 at 11:30:12AM -0800, Suren Baghdasaryan wrote:

quoted

For memory ordering (which Johannes also pointed out) the critical point is:

times[cpu] += delta           | if g->polling:
smp_wmb()                     |   g->polling = polling = 0
cmpxchg(g->polling, 0, 1)     |   smp_rmb()
                              |   delta = times[*] (through goto SLOWPATH)

So that hotpath writes to times[] then g->polling and slowpath reads
g->polling then times[]. cmpxchg() implies a full barrier, so we can
drop smp_wmb(). Something like this:

times[cpu] += delta           | if g->polling:
cmpxchg(g->polling, 0, 1)     |   g->polling = polling = 0
                              |   smp_rmb()
                              |   delta = times[*] (through goto SLOWPATH)

Would that address your concern about ordering?

cmpxchg() implies smp_mb() before and after, so the smp_wmb() on the
left column is superfluous.

Should I keep it in the comments to make it obvious and add a note
about implicit barriers being the reason we don't call smp_mb() in the
code explicitly?

The right hand column is actively wrong; because that reads like it
wants to order a store (g->polling = 0) and a load (d = times[]), and
therefore requires smp_mb().

Just to clarify, smp_mb() is needed only in the comments or do you
want an explicit smp_mb() in the code as well? As Johannes noted
get_recent_times() which is part of "delta = times[*]" operation
involves read_seqcount section that should act as implicit memory
barrier in the slowpath.

Also, you probably want to use atomic_t for g->polling, because we
(sadly) have architectures where regular stores and atomic ops don't
work 'right'.

Oh, I see. Will do. Thanks!

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