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Date: Tue, 20 Jan 2015 09:03:12 -0500
From: Peter Hurley <peter@hurleysoftware.com>
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To: paulmck@linux.vnet.ibm.com
CC: Peter Zijlstra <peterz@infradead.org>, Kent Overstreet <kmo@daterainc.com>,
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Subject: Re: Linux 3.19-rc3
References: <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> <54ABDB4B.7070008@hurleysoftware.com> <20150106173808.GB5280@linux.vnet.ibm.com> <54AC224C.4030903@hurleysoftware.com> <20150106192559.GF5280@linux.vnet.ibm.com> <54AC3E31.2080202@hurleysoftware.com> <20150106204753.GI5280@linux.vnet.ibm.com> <20150120003008.GA14445@linux.vnet.ibm.com>
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On 01/19/2015 07:30 PM, Paul E. McKenney wrote:
> On Tue, Jan 06, 2015 at 12:47:53PM -0800, Paul E. McKenney wrote:
>> On Tue, Jan 06, 2015 at 02:57:37PM -0500, Peter Hurley wrote:
> 
> [ . . . ]
> 
>> David Miller's call, actually.
>>
>> But the rule is that if it is an atomic read-modify-write operation and it
>> returns a value, then the operation itself needs to include full memory
>> barriers before and after (as in the caller doesn't need to add them).
>> Otherwise, the operation does not need to include memory ordering.
>> Since xchg(), atomic_xchg(), and atomic_long_xchg() all return a value,
>> their implementations must include full memory barriers before and after.
>>
>> Pretty straightforward.  ;-)
> 
> Hello again, Peter,
> 
> Were you going to push a patch clarifying this?

Hi Paul,

As you pointed out, atomic_ops.txt is for arch implementors, so I wasn't
planning on patching that file.

I've been meaning to write up something specifically for everyone else but
my own bugs have kept me from that. [That, and I'm not sure what I write
will be suitable for Documentation.]

Regards,
Peter Hurley