Messages in this thread |  | | Date | Fri, 6 Apr 2018 12:34:36 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 10/10] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb() |
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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
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