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Re: [Xen-devel] [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS
- To: Ingo Molnar <mingo@xxxxxxxxxx>
- From: Waiman Long <waiman.long@xxxxxx>
- Date: Fri, 18 Apr 2014 12:26:26 -0400
- Cc: linux-arch@xxxxxxxxxxxxxxx, Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>, Gleb Natapov <gleb@xxxxxxxxxx>, kvm@xxxxxxxxxxxxxxx, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Scott J Norton <scott.norton@xxxxxx>, x86@xxxxxxxxxx, Paolo Bonzini <paolo.bonzini@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, Ingo Molnar <mingo@xxxxxxxxxx>, Chegu Vinod <chegu_vinod@xxxxxx>, David Vrabel <david.vrabel@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Oleg Nesterov <oleg@xxxxxxxxxx>
- Delivery-date: Fri, 18 Apr 2014 16:26:49 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
On 04/18/2014 03:46 AM, Ingo Molnar wrote:
* Waiman Long<waiman.long@xxxxxx> wrote:
On 04/17/2014 11:58 AM, Peter Zijlstra wrote:
On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
+static __always_inline void
+clear_pending_set_locked(struct qspinlock *lock, u32 val)
+{
+ struct __qspinlock *l = (void *)lock;
+
+ ACCESS_ONCE(l->locked_pending) = 1;
+}
@@ -157,8 +251,13 @@ static inline int trylock_pending(struct qspinlock *lock,
u32 *pval)
* we're pending, wait for the owner to go away.
*
* *,1,1 -> *,1,0
+ *
+ * this wait loop must be a load-acquire such that we match the
+ * store-release that clears the locked bit and create lock
+ * sequentiality; this because not all try_clear_pending_set_locked()
+ * implementations imply full barriers.
You renamed the function referred in the above comment.
Sorry, will fix the comments.
I suggest not renaming the function instead.
try_clear_pending_set_locked() tells the intent in a clearer fashion.
Thanks,
Ingo
I usually use the word "try" if there is a possibility of failure.
However, the function will always succeed, albeit by waiting a bit in
some cases. That is why I remove "try" from the name.
-Longman
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