linux-kernel - Re: Linux 3.19-rc3

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Date:	Tue, 06 Jan 2015 07:55:39 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Kent Overstreet <kmo@...erainc.com>
CC:	Sedat Dilek <sedat.dilek@...il.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Chris Mason <clm@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Linux 3.19-rc3

[ +cc Paul McKenney ]

On 01/06/2015 07:20 AM, Peter Zijlstra wrote:
> On Tue, Jan 06, 2015 at 04:01:21AM -0800, Kent Overstreet wrote:
>> On Tue, Jan 06, 2015 at 12:48:42PM +0100, Peter Zijlstra wrote:
>>>
>>>
>>> Looking at that closure stuff, why is there an smp_mb() in
>>> closure_wake_up() ? Typically wakeup only needs to imply a wmb.
>>>
>>> Also note that __closure_wake_up() starts with a fully serializing
>>> instruction (xchg) and thereby already implies the full barrier.
>>
>> Probably no good reason, that code is pretty old :)
>>
>> If I was to hazard a guess, I had my own lockless linked lists before llist.h
>> existed and perhaps I did it with atomic_xchg() - which was at least documented
>> to not imply a barrier. I suppose it should just be dropped.
> 
> We (probably me) should probably audit all the atomic_xchg()
> implementations and documentation and fix that. I was very much under
> the impression it should imply a full barrier (and it certainly does on
> x86), the documentation should state the rule that any atomic_ function
> that returns a result is fully serializing, therefore, because
> atomic_xchg() has a return value, it should too.

memory-barriers.txt and atomic_ops.txt appear to contradict each other here,
but I think that's because atomic_ops.txt has drifted toward an
arch-implementer's POV:

260:atomic_xchg requires explicit memory barriers around the operation.

All the serializing atomic operations have descriptions like this.

Regards,
Peter Hurley
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