linux-kernel - Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan

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Message-ID: <20150501152157.GF5381@linux.vnet.ibm.com>
Date:	Fri, 1 May 2015 08:21:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jason Low <jason.low2@...com>
Cc:	Waiman Long <waiman.long@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq

On Thu, Apr 30, 2015 at 02:13:07PM -0700, Jason Low wrote:
> On Thu, 2015-04-30 at 14:42 -0400, Waiman Long wrote:
> 
> > I do have a question of what kind of tearing you are talking about. Do 
> > you mean the tearing due to mm being changed in the middle of the 
> > access? The reason why I don't like this kind of construct is that I am 
> > not sure if
> > the address translation p->mm->numa_scan_seq is being done once or 
> > twice. I looked at the compiled code and the translation is done only once.
> > 
> > Anyway, the purpose of READ_ONCE and WRITE_ONCE is not for eliminating 
> > data tearing. They are to make sure that the compiler won't compile away 
> > data access and they are done in the order they appear in the program. I 
> > don't think it is a good idea to associate tearing elimination with 
> > those macros. So I would suggest removing the last sentence in your comment.
> 
> Yes, I can remove the last sentence in the comment since the main goal
> was to document that we're access this field without exclusive access.
> 
> In terms of data tearing, an example would be the write operation gets
> split into multiple stores (though this is architecture dependent). The
> idea was that since we're modifying a seq variable without the write
> lock, we want to remove any forms of optimizations as mentioned above or
> unpredictable behavior, since READ_ONCE/WRITE_ONCE adds no overhead.

Just to be clear...  READ_ONCE() and WRITE_ONCE() do not avoid data tearing
in cases where the thing read or written is too big for a machine word.
If the thing read/written does fit into a machine word and if the location
read/written is properly aligned, I would be quite surprised if either
READ_ONCE() or WRITE_ONCE() resulted in any sort of tearing.

							Thanx, Paul

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