Re: [Xen-devel] [PATCH v15 12/15] pvqspinlock, x86: Enable PV qspinlock for Xen

From: Waiman Long
Date: Wed Apr 08 2015 - 13:43:10 EST

On 04/08/2015 08:01 AM, David Vrabel wrote:
On 07/04/15 03:55, Waiman Long wrote:
This patch adds the necessary Xen specific code to allow Xen to
support the CPU halting and kicking operations needed by the queue
spinlock PV code.
This basically looks the same as the version I wrote, except I think you
broke it.

+static void xen_qlock_wait(u8 *byte, u8 val)
+ int irq = __this_cpu_read(lock_kicker_irq);
+ /* If kicker interrupts not initialized yet, just spin */
+ if (irq == -1)
+ return;
+ /* clear pending */
+ xen_clear_irq_pending(irq);
+ /*
+ * We check the byte value after clearing pending IRQ to make sure
+ * that we won't miss a wakeup event because of the clearing.
My version had a barrier() here to ensure this. The documentation of
READ_ONCE() suggests that it is not sufficient to meet this requirement
(and a READ_ONCE() here is not required anyway).

I have the misconception that a READ_ONCE/WRITE_ONCE() implies a compiler barrier. You are right that it may not be the case. I will add back the missing barrier when I update the patch and add you as the author of this patch if you don't mind.

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