linux-kernel - Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl

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Date:	Tue, 7 Jun 2016 14:41:44 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Will Deacon <will.deacon@....com>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Waiman Long <waiman.long@....com>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	manfred@...orfullife.com, dave@...olabs.net, boqun.feng@...il.com,
	tj@...nel.org, pablo@...filter.org, kaber@...sh.net,
	davem@...emloft.net, oleg@...hat.com,
	netfilter-devel@...r.kernel.org, sasha.levin@...cle.com,
	hofrat@...dl.org
Subject: Re: [RFC][PATCH 1/3] locking: Introduce smp_acquire__after_ctrl_dep

On 07.06.2016 09:15, Peter Zijlstra wrote:
>>
>> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
>> index 147ae8ec836f..a4d0a99de04d 100644
>> --- a/Documentation/memory-barriers.txt
>> +++ b/Documentation/memory-barriers.txt
>> @@ -806,6 +806,41 @@ out-guess your code.  More generally, although READ_ONCE() does force
>>  the compiler to actually emit code for a given load, it does not force
>>  the compiler to use the results.
>>  
>> +In addition, control dependencies apply only to the then-clause and
>> +else-clause of the if-statement in question.  In particular, it does
>> +not necessarily apply to code following the if-statement:
>> +
>> +	q = READ_ONCE(a);
>> +	if (q) {
>> +		WRITE_ONCE(b, p);
>> +	} else {
>> +		WRITE_ONCE(b, r);
>> +	}
>> +	WRITE_ONCE(c, 1);  /* BUG: No ordering against the read from "a". */
>> +
>> +It is tempting to argue that there in fact is ordering because the
>> +compiler cannot reorder volatile accesses and also cannot reorder
>> +the writes to "b" with the condition.  Unfortunately for this line
>> +of reasoning, the compiler might compile the two writes to "b" as
>> +conditional-move instructions, as in this fanciful pseudo-assembly
>> +language:

I wonder if we already guarantee by kernel compiler settings that this
behavior is not allowed by at least gcc.

We unconditionally set --param allow-store-data-races=0 which should
actually prevent gcc from generating such conditional stores.

Am I seeing this correct here?

Thanks,
Hannes