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

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Message-ID: <20151016163915.GB3816@twins.programming.kicks-ass.net>
Date:	Fri, 16 Oct 2015 18:39:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: Q: schedule() and implied barriers on arm64

On Fri, Oct 16, 2015 at 09:28:24AM -0700, Paul E. McKenney wrote:

> > Maybe some previous RCU variant relied on this?
> 
> Yes, older versions did rely on this.  Now, only the CPU itself observes
> RCU's state changes during context switch.  I couldn't tell you exactly
> when this changed.  :-/
> 
> With the exception of some synchronize_sched_expedited() cases, but in
> those cases, RCU code acquires the CPU's leaf rcu_node structure's
> ->lock, and with the required strong transitivity.

OK, so I can scrap this 'requirement' from my list. All sorted, thanks!

> > > Well, arm64 might well need smp_mb__after_unlock_lock() to be non-empty.
> > 
> > Its UNLOCK+LOCK should be RCsc, so that should be good. Its just that
> > LOCK+UNLOCK isn't anything.
> 
> Ah!  If RCU relies on LOCK+UNLOCK being a barrier of any sort, that is a
> bug in RCU that needs fixing.

Don't think RCU does that, But its what schedule() provides in the
weakest case. Hence my question here.
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