linux-kernel - Re: [PATCH 4/4] exit: Lockless iteration over task list in mm_update_next

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Message-ID: <20180426152000.GB4082@hirez.programming.kicks-ass.net>
Date:   Thu, 26 Apr 2018 17:20:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        akpm@...ux-foundation.org, oleg@...hat.com,
        viro@...iv.linux.org.uk, mingo@...nel.org,
        paulmck@...ux.vnet.ibm.com, keescook@...omium.org, riel@...hat.com,
        mhocko@...e.com, tglx@...utronix.de,
        kirill.shutemov@...ux.intel.com, marcos.souza.org@...il.com,
        hoeun.ryu@...il.com, pasha.tatashin@...cle.com, gs051095@...il.com,
        ebiederm@...ssion.com, dhowells@...hat.com,
        rppt@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 4/4] exit: Lockless iteration over task list in
 mm_update_next_owner()

On Thu, Apr 26, 2018 at 04:52:39PM +0300, Kirill Tkhai wrote:
> In the patch I used the logic, that the below code:
> 
> 	x = A;
> 	spin_lock();
> 	spin_unlock();
> 	spin_lock();
> 	spin_unlock();
> 	y = B;
> 
> cannot reorder much than:
> 
> 	spin_lock();
> 	x = A;		<- this can't become visible later, that spin_unlock()
> 	spin_unlock();
> 	spin_lock();
> 	y = B;		<- this can't become visible earlier, than spin_lock()
> 	spin_unlock();
> 
> Is there a problem?

The two stores will be ordered, but only at the strength of an
smp_wmb(). The above construct does not imply smp_mb(). The difference
is observable on real hardware (Power).