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SubjectRe: [v3,11/41] mips: reuse asm-generic/barrier.h
Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>
> You could use SYNC_ACQUIRE() to implement read_barrier_depends() and
> smp_read_barrier_depends(), but SYNC_RMB probably does not suffice.
> The reason for this is that smp_read_barrier_depends() must order the
> pointer load against any subsequent read or write through a dereference
> of that pointer. For example:
>
> p = READ_ONCE(gp);
> smp_rmb();
> r1 = p->a; /* ordered by smp_rmb(). */
> p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */
> r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */
>
> In contrast:
>
> p = READ_ONCE(gp);
> smp_read_barrier_depends();
> r1 = p->a; /* ordered by smp_read_barrier_depends(). */
> p->b = 42; /* ordered by smp_read_barrier_depends(). */
> r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */
>
> Again, if your hardware maintains local ordering for address
> and data dependencies, you can have read_barrier_depends() and
> smp_read_barrier_depends() be no-ops like they are for most
> architectures.
>
> Does that help?

This is crazy! smp_rmb started out being strictly stronger than
smp_read_barrier_depends, when did this stop being the case?
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

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