On Sat, 21 Oct 2006, Paul E. McKenney wrote:
> > This is identical to the previous version, since by definition
> >
> > st_i(B) ==> ld_j(B) is equivalent to st_i(B) => ld_j(B) &&
> > not exist k: st_i(B) => st_k(B) => ld_j(B).
>
> OK -- we were assuming slightly different definitions of "==>". I as
> assuming that if st==>ld1==>ld2, that it is not the case that "st==>ld2".
> In this circumstance, your definition is certainly more convenient than
> is mine. In the case of MMIO, the situation might be reversed.
MMIO of course is completely different. For regular memory accesses I
think we should never allow a load on the left side of "=>" or "==>".
Keep them invisible! :-)
Writing ld(A) => st(A) is bad because (1) it suggests that the store
somehow "sees" the load (which it doesn't; the load is invisible), and (2)
it suggests that the store occurs "later" in some sense than the load
(which might not be true, since a load doesn't necessarily return the
value of the temporally most recent store).
My viewpoint is that "=>" really provides an ordering of stores only.
Its use with loads is something of an artifact; it gives a convenient way
of expressing the fact that a load "sees" an initial segment of all the
stores to a variable (and the value it returns is that of the last store
in the segment).
> > (2) doesn't make sense, since loads aren't part of the global ordering of
> > accesses of B -- they are invisible. (BTW, you don't need to assume as
> > well that stores are blind; it's enough just to have loads be invisible.)
> > Each load sees an initial sequence of stores ending in the store whose
> > value is returned by the load, but this doesn't mean that the load occurs
> > between that store and the next one.
>
> That is due to our difference in definition. Perhaps the following
> definition: "A==>B" means either that B sees the value stored by A
> or that B sees the same value as does A?
>
> Some work will be required to see what is best.
How about this instead: "A==>B" means that B sees the value stored by A,
and "A==B" means that A and B are both loads and they see the value from
the same store. That way we avoid putting a load on the left side of
"==>".
> > (3) The assumption should be that both accesses of B are atomic; it
> > doesn't matter whether the accesses of A are.
>
> Check out the i386 default definition of spin_unlock() -- no atomic
> operations. So only the final access of B (the one corresponding to
> spin_lock()) would need to be atomic.
You are right.
Alan
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
