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

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Date:	Thu, 5 Dec 2013 14:47:10 -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 10:05:47AM -0800, Paul E. McKenney wrote:
> 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.

Which reminds me -- the thing that makes me most nervous about prohibiting
speculative stores is the bit about having to anticipate all compiler
optimizations that might get rid of the needed conditionals.

Thoughts?

							Thanx, Paul

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