linux-kernel - Q: schedule() and implied barriers on arm64

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Date:	Fri, 16 Oct 2015 17:18:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Q: schedule() and implied barriers on arm64

Hi,

IIRC Paul relies on schedule() implying a full memory barrier with
strong transitivity for RCU.

If not, ignore this email.

If so, however, I suspect AARGH64 is borken and would need (just like
PPC):

#define smp_mb__before_spinlock()	smp_mb()

The problem is that schedule() (when a NO-OP) does:

	smp_mb__before_spinlock();
	LOCK rq->lock

	clear_bit()

	UNLOCK rq->lock

And nothing there implies a full barrier on AARGH64, since
smp_mb__before_spinlock() defaults to WMB, LOCK is an "ldaxr" or
load-acquire, UNLOCK is "stlrh" or store-release and clear_bit() isn't
anything.

Pretty much every other arch has LOCK implying a full barrier, either
because its strongly ordered or because it needs one for the ACQUIRE
semantics.

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