Re: [Patch v3] rwsem: fix rwsem_is_locked() bugs

David Howells
2009-10-07T12:20:13Z

Amerigo Wang <amwang@redhat.com> wrote:

> static inline int rwsem_is_locked(struct rw_semaphore *sem)
> {
> - return (sem->activity != 0);
> + int ret;
> +
> + if (spin_trylock_irq(&sem->wait_lock)) {
> + ret = !(list_empty(&sem->wait_list) && sem->activity == 0);
> + spin_unlock_irq(&sem->wait_lock);
> + return ret;
> + }
> + return 1;
> }

Yep... This seems a reasonable approach, though I contend that if you're
holding the spinlock, then sem->wait_list _must_ be empty if sem->activity is
0 - so that half of the test is redundant.

sem->activity == 0 and sem->wait_list not being empty is a transitional state
that can only occur in ups and downgrades whilst they hold the spinlock.

> diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
> index 9df3ca5..234d83f 100644
> --- a/lib/rwsem-spinlock.c
> +++ b/lib/rwsem-spinlock.c
> @@ -78,7 +78,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
>
> /* grant an infinite number of read locks to the front of the queue */
> dont_wake_writers:
> - woken = 0;
> + /*
> + * we increase ->activity just to make rwsem_is_locked() happy,
> + * to avoid potential cache line ping-pong, we don't do this
> + * within the following loop.
> + */
> + woken = sem->activity++;
> while (waiter->flags & RWSEM_WAITING_FOR_READ) {
> struct list_head *next = waiter->list.next;
>
> @@ -94,7 +99,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
> waiter = list_entry(next, struct rwsem_waiter, list);
> }
>
> - sem->activity += woken;
> + sem->activity = woken;
>
> out:
> return sem;

This change to __rwsem_do_wake() is all unnecessary - you're defending against
the test of sem->activity by rwsem_is_locked() - but that now happens with the
spinlock held.

David