linux-kernel - Re: [RFC PATCH v3 1/2] membarrier: Provide register expedited private command

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Date:   Mon, 25 Sep 2017 12:42:48 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        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 v3 1/2] membarrier: Provide register expedited
 private command

----- On Sep 25, 2017, at 8:25 AM, Peter Zijlstra peterz@...radead.org wrote:

> On Mon, Sep 25, 2017 at 08:10:54PM +0800, Boqun Feng wrote:
>> > static void membarrier_register_private_expedited(void)
>> > {
>> >         struct task_struct *p = current;
>> > 
>> >         if (READ_ONCE(p->mm->membarrier_private_expedited))
>> >                 return;
>> >         membarrier_arch_register_private_expedited(p);
> 
> Should we not then also do:
> 
>	    barrier();
> 
>> >         WRITE_ONCE(p->mm->membarrier_private_expedited, 1);
>> > }
> 
> to avoid the compiler lifting that store?

membarrier_arch_register_private_expedited() being a function call, I
recall compilers cannot move load/stores across those. Moreover, even if
that function would happen to be eventually inlined, synchronize_sched()
is needed at the end of the function to ensure the scheduler will observe
the thread flags before it returns. That too would then act as a compiler
barrier if that function is ever inlined in the future.

So do you think we should still add the barrier() as documentation, or is
having synchronize_sched() in the callee enough ?

By the way, I think I should add a READ_ONCE() in membarrier_private_expedited
to pair with the WRITE_ONCE() in registration, such as:

        if (!READ_ONCE(current->mm->membarrier_private_expedited))
                return -EPERM;

Thanks!

Mathieu



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