linux-kernel - Re: [PATCH tip/core/locking 4/4] Documentation/memory-barriers.txt: Document ACCESS

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Message-ID: <20131205180547.GU15492@linux.vnet.ibm.com>
Date:	Thu, 5 Dec 2013 10:05:47 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	Oleg Nesterov <oleg@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH tip/core/locking 4/4] Documentation/memory-barriers.txt:
 Document ACCESS_ONCE()

On Thu, Dec 05, 2013 at 10:50:42AM +0100, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > + (*) The compiler is within its rights to reload a variable, for example,
> > +     in cases where high register pressure prevents the compiler from
> > +     keeping all data of interest in registers.  The compiler might
> > +     therefore optimize the variable tmp out of our previous example:
> > +
> > +	while (tmp = a)
> > +		do_something_with(tmp);
> > +
> > +     This could result in the following code, which is perfectly safe in
> > +     single-threaded code, but can be fatal in concurrent code:
> > +
> > +	while (a)
> > +		do_something_with(a);
> > +
> > +     For example, the optimized version of this code could result in
> > +     passing a zero to do_something_with() in the case where the variable
> > +     a was modified by some other CPU between the "while" statement and
> > +     the call to do_something_with().
> 
> Nit: I guess references to variable names such as 'a' should be quoted 
> (same for 'tmp', 'b', etc):
> 
>         For example, the optimized version of this code could result in
>         passing a zero to do_something_with() in the case where the variable
>         'a' was modified by some other CPU between the "while" statement and
>         the call to do_something_with().
> 
> which makes reading it less ambiguous and more fluid IMO. This 
> observation applies to the whole document as 'a' is used in many 
> places.

Good point, fixed.

							Thanx, Paul

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