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

Amerigo Wang
2009-10-08T09:13:55Z

David Howells wrote:
> 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.
>

Hmm, yeah...

>> 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.

Ah, yes, I knew this, I kept this just for completeness.
I will remove this part then. :)

THanks!