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

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Date:	Fri, 01 May 2015 10:40:08 -0700
From:	Jason Low <jason.low2@...com>
To:	paulmck@...ux.vnet.ibm.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>, jason.low2@...com
Subject: Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq

On Fri, 2015-05-01 at 08:21 -0700, Paul E. McKenney wrote:
> 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.

Right, that makes sense. I've updated the comment to instead mention
that it's used to avoid "compiler optimizations".

> 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.

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