Re: [Xen-devel] [PATCH v9 05/19] qspinlock: Optimize for smaller NR_CPUS

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
> +struct __qspinlock {
> +     union {
> +             atomic_t val;
> +             struct {
> +#ifdef __LITTLE_ENDIAN
> +                     u16     locked_pending;
> +                     u16     tail;
> +#else
> +                     u16     tail;
> +                     u16     locked_pending;
> +#endif
> +             };
> +     };
> +};
> +
> +/**
> + * clear_pending_set_locked - take ownership and clear the pending bit.
> + * @lock: Pointer to queue spinlock structure
> + * @val : Current value of the queue spinlock 32-bit word
> + *
> + * *,1,0 -> *,0,1
> + */
> +static __always_inline void
> +clear_pending_set_locked(struct qspinlock *lock, u32 val)
> +{
> +     struct __qspinlock *l = (void *)lock;
> +
> +     ACCESS_ONCE(l->locked_pending) = 1;

You lost the __constant_le16_to_cpu(_Q_LOCKED_VAL) there. The
unconditional 1 is wrong. You also have to flip the bytes in
locked_pending.

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