On Wed, Jan 16, 2019 at 11:27 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Wed, Jan 16, 2019 at 02:17:28PM -0500, Johannes Weiner wrote: > > 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() > > Actually, I might have been mistaken about this. The seqcount locking > does an smp_rmb() and an smp_wmb(), and that orders reads and writes > respectively, but doesn't necessarily order reads against writes. > > So I think we need an explicit smp_mb() after all. I see. So, the action items I collected so far: 1. Add a comment in the code next to cmpxchg() indicating implicit smp_mb. 2. Add explicit smp_mb after "g->polling = 0" and before "delta = times[*]" both in the code and in the comments (in the slowpath). 3. Use atomic_t for g->polling. Add a note in the comments why atomic operations are not needed in the slowpath. 4. Minimize line-breaks. Please let me know if I missed anything, otherwise will make these changes and post ver 3 of the patchset. Thanks, Suren.
