On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
> +/*
> + * Try to acquire write lock before the writer has been put on wait queue.
> + */
> +static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> +{
> + long count = ACCESS_ONCE(sem->count);
> +retry:
> + if (count == RWSEM_WAITING_BIAS) {
> + count = cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS);
> + /* allow write lock stealing, try acquiring the write lock. */
> + if (count == RWSEM_WAITING_BIAS)
> + goto acquired;
> + else if (count == 0)
> + goto retry;
> + } else if (count == 0) {
> + count = cmpxchg(&sem->count, 0, RWSEM_ACTIVE_WRITE_BIAS);
> + if (count == 0)
> + goto acquired;
> + else if (count == RWSEM_WAITING_BIAS)
> + goto retry;
> + }
> + return false;
> +
> +acquired:
> + return true;
> +}
Could we have written that like:
static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
{
long old, count = ACCESS_ONCE(sem->count);
for (;;) {
if (!(count == 0 || count == RWSEM_WAITING_BIAS))
return false;
old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_BIAS);
if (old == count)
return true;
count = old;
}
}
?
Just checking my brain got all the branches untangled right :-)
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