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:

quoted

On Thu, Jun 28, 2018 at 12:41:19PM +0200, Andrea Parri wrote:

quoted

- * 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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