[PATCH] Document Linux's memory barriers

David Howells dhowells at redhat.com
Thu Mar 9 07:16:11 EST 2006

Pavel Machek <pavel at ucw.cz> wrote:

> > + (*) set_mb(var, value)
> > + (*) set_wmb(var, value)
> > +
> > +     These assign the value to the variable and then insert at least a write
> > +     barrier after it, depending on the function.
> > +
> I... don't understand what these do. Better explanation would
> help.. .what is function?

I can only guess, and hope someone corrects me if I'm wrong.

> Does it try to say that set_mb(var, value) is equivalent to var =
> value; mb();


> but here mb() affects that one variable, only?

No. set_*mb() is simply a canned sequence of assignment, memory barrier.

The type of barrier inserted depends on which function you choose. set_mb()
inserts an mb() and set_wmb() inserts a wmb().

> "LOCK access"?

The LOCK and UNLOCK functions presumably make at least one memory write apiece
to manipulate the target lock (on SMP at least).

> Does it try to say that ...will be completed after any access inside lock
> region is completed?

No. What you get in effect is something like:

	LOCK { *lock = q; }
	*A = a;
	*B = b;
	UNLOCK { *lock = u; }

Except that the accesses to the lock memory are made using special procedures
(LOCK prefixed instructions, XCHG, CAS/CMPXCHG, LL/SC, etc).

> This makes it sound like pentium-III+ is incompatible with previous
> CPUs. Is it really the case?

Yes - hence the alternative instruction stuff.


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