Re: [PATCH] make atomic_t volatile on all architectures

[Chris Snook <csnook@redhat.com>]

Linus Torvalds wrote:
> On Wed, 8 Aug 2007, Chris Snook wrote:
> > Some architectures currently do not declare the contents of an atomic_t to be
> > volatile.  This causes confusion since atomic_read() might not actually read
> > anything if an optimizing compiler re-uses a value stored in a register, which
> > can break code that loops until something external changes the value of an
> > atomic_t.
> I'd be *much* happier with "atomic_read()" doing the "volatile" instead.
>
> The fact is, volatile on data structures is a bug. It's a wart in the C 
> language. It shouldn't be used. 
>
> Volatile accesses in *code* can be ok, and if we have "atomic_read()" 
> expand to a "*(volatile int *)&(x)->value", then I'd be ok with that.
> But marking data structures volatile just makes the compiler screw up 
> totally, and makes code for initialization sequences etc much worse.
> 		Linus
Fair enough.  Casting to (volatile int *) will give us the behavior people 
expect when using atomic_t without needing to use inefficient barriers.
While we have the hood up, should we convert all the atomic_t's to non-volatile 
and put volatile casts in all the atomic_reads?  I don't know enough about the 
various arches to say with confidence that those changes alone will preserve 
existing behavior.  We might need some arch-specific tweaking of the atomic 
operations.
	-- Chris
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/