linux-kernel - Re: [RFC PATCH v2 1/2] membarrier: Provide register expedited private command

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Date:   Thu, 21 Sep 2017 15:47:19 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        maged michael <maged.michael@...il.com>,
        gromer <gromer@...gle.com>, Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC PATCH v2 1/2] membarrier: Provide register expedited
 private command

----- On Sep 21, 2017, at 11:27 AM, Paul E. McKenney paulmck@...ux.vnet.ibm.com wrote:

> On Thu, Sep 21, 2017 at 03:11:43PM +0000, Mathieu Desnoyers wrote:
>> ----- On Sep 21, 2017, at 8:13 AM, Peter Zijlstra peterz@...radead.org wrote:
>> 
>> > On Mon, Sep 18, 2017 at 06:36:47PM -0400, Mathieu Desnoyers wrote:
>> >> @@ -3373,6 +3362,7 @@ static void __sched notrace __schedule(bool preempt)
>> >>  
>> >>  		/* Also unlocks the rq: */
>> >>  		rq = context_switch(rq, prev, next, &rf);
>> >> +		membarrier_arch_sched_in(prev, next);
>> > 
>> > That's wrong iirc; we just switched stacks, you cannot use @prev and
>> > @next.
>> 
>> Yes, it did indeed explode spectacularly at boot in testing. RFC v3 fixes that.
> 
> Good to hear that there is a useful test.  ;-)

For the issue identified by PeterZ, just booting the machine was enough to
touch a task_struct after it was put, which exploded. After fixing this, I
ran through the membarrier selftests: they run fine.

Then I did a test branch for liburcu [1] which uses the private cmd with
registration. Both 'make check' and 'make regtest' run fine on Power8.

Thanks,

Mathieu

[1] https://github.com/compudj/userspace-rcu-dev/tree/test-urcu-reg-priv

> 
>							Thanx, Paul
> 
>> Thanks,
>> 
>> Mathieu
>> 
>> 
>> > 
>> >>  	} else {
>> >>  		rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
>> > >  		rq_unlock_irq(rq, &rf);
>> 
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com