Messages in this thread | | | Date | Mon, 2 Jul 2018 14:50:03 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/3] locking: Clarify requirements for smp_mb__after_spinlock() |
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On Thu, Jun 28, 2018 at 07:30:45PM +0200, Andrea Parri wrote: > On Thu, Jun 28, 2018 at 05:05:50PM +0200, Peter Zijlstra wrote: > > On Thu, Jun 28, 2018 at 12:41:19PM +0200, Andrea Parri wrote: > > > - * This barrier must provide two things: > > > - * > > > - * - it must guarantee a STORE before the spin_lock() is ordered against a > > > - * LOAD after it, see the comments at its two usage sites. > > > - * > > > - * - it must ensure the critical section is RCsc. > > > - * > > > - * The latter is important for cases where we observe values written by other > > > - * CPUs in spin-loops, without barriers, while being subject to scheduling. > > > - * > > > - * CPU0 CPU1 CPU2 > > > - * > > > - * for (;;) { > > > - * if (READ_ONCE(X)) > > > - * break; > > > - * } > > > - * X=1 > > > - * <sched-out> > > > - * <sched-in> > > > - * r = X; > > > - * > > > - * without transitivity it could be that CPU1 observes X!=0 breaks the loop, > > > - * we get migrated and CPU2 sees X==0. > > > > Please don't remove that; that explains _why_ we need a full memory > > barrier here. > > Peter: > > Both you and Boqun stated that the above snippet is "bad": > > http://lkml.kernel.org/r/20180312085646.GE4064@hirez.programming.kicks-ass.net > > and I do agree with your assessment! ;-)
Right..
> I've no objection to keep that comment (together with the > "clarification" suggested in this patch) _once_ replaced > that snippet with something else (say, with the snippet > Boqun suggested in: > > http://lkml.kernel.org/r/20180312085600.aczjkpn73axzs2sb@tardis ): > > is this what you mean?
Yes. I much prefer to explain the why for rule than to just state them.
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