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Date: Tue, 6 Jan 2015 13:20:06 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: 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>
Subject: Re: Linux 3.19-rc3
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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.