lkml.org 
[lkml]   [2018]   [Apr]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 10/10] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb()
On Thu, Apr 05, 2018 at 07:28:08PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 05, 2018 at 05:59:07PM +0100, Will Deacon wrote:
> > @@ -340,12 +341,17 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > goto release;
> >
> > /*
> > + * Ensure that the initialisation of @node is complete before we
> > + * publish the updated tail and potentially link @node into the
> > + * waitqueue.
> > + */
> > + smp_wmb();
>
> Maybe an explicit note to where the matching barrier lives..

Oh man, that's not a simple thing to write: there isn't a matching barrier!

Instead, we rely on dependency ordering for two cases:

* We access a node by decoding the tail we get back from the xchg

- or -

* We access a node by following our own ->next pointer

I could say something like:

"Pairs with dependency ordering from both xchg_tail and explicit
dereferences of node->next"

but it's a bit cryptic :(

Will

\
 
 \ /
  Last update: 2018-04-06 13:34    [W:0.130 / U:0.876 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site