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

[Alan Stern]

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

> No sane compiler would change it to a byte-at-a-time store, but the
> compiler would nevertheless be within its rights to do so.  And a quick
> review of certain LKML threads could easily cause anyone to question gcc's
> sanity.  Furthermore, the compiler is permitted to make transformations
> like the following, which it might well do to save a branch:
> 
> 	if (b)				a = 0;
> 		a = 1;			if (b)
> 	else					a = 1;
> 		a = 0;

The compiler would be forbidden if the original code were

	if (b)
		ACCESS_ONCE(a) = 1;
	else
		ACCESS_ONCE(a) = 0;

But if I remember correctly, the code snippet we were talking was more 
like:

	if (ACCESS_ONCE(b))
		a = 1;

Isn't this use of ACCESS_ONCE unnecessary?

> In short, without ACCESS_ONCE(), the compiler is within its rights to
> assume that the code is single-threaded.  There are a large number of
> non-obvious optimizations that are safe in single-threaded code but
> destructive in multithreaded code.

Followed to its logical conclusion, this means that virtually every
access to a shared variable that isn't protected by some sort of lock
or isn't one of the special atomic operations needs to use
ACCESS_ONCE.  The kernel must be riddled with places that don't do 
this.

Besides, how sure are you that even in the presence of ACCESS_ONCE, the
compiler will not make any unsafe transformations?  For example, is the
compiler forbidden from transforming

	if (a)
		ACCESS_ONCE(b) = 5;

into

	tmp = c;
	c = 999;
	if (a)
		ACCESS_ONCE(b) = 5;
	c = tmp;

?  After all, the c variable isn't protected by ACCESS_ONCE in the 
original code.  And yet this transformation is clearly _unsafe_ for 
multi-threaded operation.

> In addition, and perhaps more important, ACCESS_ONCE() serves as useful
> documentation of the fact that the variable is accessed outside of locks.

True.

Alan Stern



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