On Wed, Jan 16, 2019 at 09:39:13AM -0800, Suren Baghdasaryan wrote: > On Wed, Jan 16, 2019 at 5:24 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > On Mon, Jan 14, 2019 at 11:30:12AM -0800, Suren Baghdasaryan wrote: > > > 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? I'd keep 'em out if they aren't actually in the code. But I'd switch delta = times[*] in this comment to to get_recent_times() // implies smp_mb() or something to make the ordering a bit more visible. And also add a comment to the actual cmpxchg() in the code directly that says that we rely on the implied ordering and that it pairs with the smp_mb() in the slowpath; add a similar comment to the smp_mb(). > > 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! Yikes, that's news to me too. Good to know.
