Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH] Document Linux's memory barriers | Date | Wed, 08 Mar 2006 20:16:11 +0000 |
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Pavel Machek <pavel@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();
Yes.
> 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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