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

[David Vrabel <david.vrabel@citrix.com>]

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).

> +	 *
> +	 * The sync_clear_bit() call in xen_clear_irq_pending() is atomic.
> +	 * So it is effectively a memory barrier for x86.
> +	 */
> +	if (READ_ONCE(*byte) != val)
> +		return;
> +
> +	/*
> +	 * If an interrupt happens here, it will leave the wakeup irq
> +	 * pending, which will cause xen_poll_irq() to return
> +	 * immediately.
> +	 */
> +
> +	/* Block until irq becomes pending (or perhaps a spurious wakeup) */
> +	xen_poll_irq(irq);
> +}

David