Re: [PATCH] driver core: fix shutdown races with probe/remove

[Alan Stern]

On Wed, 6 Jun 2012, Paul E. McKenney wrote:

> On Wed, Jun 06, 2012 at 11:44:50AM -0400, Alan Stern wrote:
> > On Wed, 6 Jun 2012, Paul E. McKenney wrote:
> > 
> > > > That just seems wrong.  By the same reasoning, the compiler is within 
> > > > its rights to transform either the original code or the code using 
> > > > ACCESS_ONCE into:
> > > > 
> > > > 	b = 999;
> > > > 	if (a)
> > > > 		b = 9;
> > > > 	else
> > > > 		b = 42;
> > > > 
> > > > and again, other code would be confused.  The simple fact is that 
> > > > SMP-safe code is not likely to be produced by a compiler that assumes 
> > > > everything is single-threaded.
> > > 
> > > If you use ACCESS_ONCE(), the compiler is prohibited from inserting
> > > the "b = 999".
> > 
> > What prohibits it?
> 
> The compiler cannot move a volatile access across a sequence point, for
> example, across a statement boundary.

How does inserting a store to a non-volatile value qualify as moving a
volatile access?

Alan Stern

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