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    SubjectRe: [PATCH] Document Linux's memory barriers
    Date
    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.

    David
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