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Message-ID: <54ABDB4B.7070008@hurleysoftware.com>
Date: Tue, 06 Jan 2015 07:55:39 -0500
From: Peter Hurley <peter@hurleysoftware.com>
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To: Peter Zijlstra <peterz@infradead.org>, Kent Overstreet <kmo@daterainc.com>
CC: Sedat Dilek <sedat.dilek@gmail.com>, Dave Jones <davej@codemonkey.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, Chris Mason <clm@fb.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: Linux 3.19-rc3
References: <20150106094039.GI29390@twins.programming.kicks-ass.net> <CA+icZUUcgP=nUPCOfEJrbO9OAAPLgQd8RWO-BJe+W0wi3fWiOw@mail.gmail.com> <CA+icZUUThdtXkRztRxk+zEqvMwUmEQeSoR3WD3qqXW9QspBoVw@mail.gmail.com> <20150106100621.GL29390@twins.programming.kicks-ass.net> <CA+icZUVy2rG3PwKhPTQojLEH2O8FD-0myO2sN8hQV4_+r9YNQQ@mail.gmail.com> <20150106110112.GQ29390@twins.programming.kicks-ass.net> <20150106110730.GA25846@kmo-pixel> <20150106114215.GS29390@twins.programming.kicks-ass.net> <20150106114842.GP10476@twins.programming.kicks-ass.net> <20150106120121.GB26845@kmo-pixel> <20150106122006.GW29390@twins.programming.kicks-ass.net>
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[ +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