linux-kernel - Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount

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Message-ID: <20150612070827.GN3644@twins.programming.kicks-ass.net>
Date:	Fri, 12 Jun 2015 09:08:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	umgwanakikbuti@...il.com, mingo@...e.hu, ktkhai@...allels.com,
	rostedt@...dmis.org, tglx@...utronix.de, juri.lelli@...il.com,
	pang.xunlei@...aro.org, oleg@...hat.com,
	wanpeng.li@...ux.intel.com, linux-kernel@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier()

On Thu, Jun 11, 2015 at 02:45:57PM -0700, Paul E. McKenney wrote:
> Color me slow and stupid.  Maybe due to reviewing a patch too early in
> the morning, who knows?
> 
> There is nothing above that prevents the compiler and the CPU from
> reordering the assignments to X and Y with the increment of s->sequence++.

That's actually fine. As long as we observe an odd value the read side
will repeat.

> Of course, this assumes that the accesses surrounding the call to
> raw_write_seqcount_barrier() are writes.  

Which is why its got both write and barrier in the name :-)
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