Messages in this thread | | | Date | Tue, 7 Jun 2016 20:45:53 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH -v4 5/7] locking, arch: Update spin_unlock_wait() |
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On Tue, Jun 07, 2016 at 02:00:16PM +0200, Peter Zijlstra wrote: > On Tue, Jun 07, 2016 at 07:43:15PM +0800, Boqun Feng wrote: > > On Mon, Jun 06, 2016 at 06:08:36PM +0200, Peter Zijlstra wrote: > > > diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c > > > index ce2f75e32ae1..e1c29d352e0e 100644 > > > --- a/kernel/locking/qspinlock.c > > > +++ b/kernel/locking/qspinlock.c > > > @@ -395,6 +395,8 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) > > > * pending stuff. > > > * > > > * p,*,* -> n,*,* > > > + * > > > + * RELEASE, such that the stores to @node must be complete. > > > */ > > > old = xchg_tail(lock, tail); > > > next = NULL; > > > @@ -405,6 +407,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) > > > */ > > > if (old & _Q_TAIL_MASK) { > > > prev = decode_tail(old); > > > + /* > > > + * The above xchg_tail() is also load of @lock which generates, > > > + * through decode_tail(), a pointer. > > > + * > > > + * The address dependency matches the RELEASE of xchg_tail() > > > + * such that the access to @prev must happen after. > > > + */ > > > + smp_read_barrier_depends(); > > > > Should this barrier be put before decode_tail()? Because it's the > > dependency old -> prev that we want to protect here. > > I don't think it matters one way or the other. The old->prev > transformation is pure; it doesn't depend on any state other than old. >
But wouldn't the old -> prev transformation get broken on Alpha semantically in theory? Because Alpha could reorder the LOAD part of the xchg_tail() and decode_tail(), which results in prev points to other cpu's mcs_node.
Though, this is fine in current code, because xchg_release() is actually xchg() on Alpha, and Alpha doesn't use qspinlock.
> I put it between prev and dereferences of prev, because that's what made > most sense to me; but really anywhere between the load of @old and the > first dereference of @prev is fine I suspect.
I understand the barrier here serves more for a documentation purpose, and I don't want to a paranoid ;-) I'm fine with the current place, just thought we could put it at somewhere more conforming to our memory model.
/me feel myself a paranoid anyway...
Regards, Boqun [unhandled content-type:application/pgp-signature] | |