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Date:   Thu, 26 Apr 2018 17:20:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     Andrea Parri <andrea.parri@amarulasolutions.com>,
        akpm@linux-foundation.org, oleg@redhat.com,
        viro@zeniv.linux.org.uk, mingo@kernel.org,
        paulmck@linux.vnet.ibm.com, keescook@chromium.org, riel@redhat.com,
        mhocko@suse.com, tglx@linutronix.de,
        kirill.shutemov@linux.intel.com, marcos.souza.org@gmail.com,
        hoeun.ryu@gmail.com, pasha.tatashin@oracle.com, gs051095@gmail.com,
        ebiederm@xmission.com, dhowells@redhat.com,
        rppt@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
        Alan Stern <stern@rowland.harvard.edu>,
        Will Deacon <will.deacon@arm.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH 4/4] exit: Lockless iteration over task list in
 mm_update_next_owner()
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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).