Re: [PATCH] rtmutex: ensure only the top waiter or higher priority task can take the lock and reduce unrelated boosting

Lai Jiangshan
2010-12-23T09:05:50Z

On 12/15/2010 11:04 PM, Steven Rostedt wrote:
> On Wed, 2010-12-15 at 16:09 +0800, Lai Jiangshan wrote:
>
> Some English updates.

Updated, Thank you very much.

>
>>

[...]

>> + * owner bit0
>> + * NULL 0 lock is free (fast acquire possible)
>> + * NULL 1 lock is free and has waiters and the top waiter
>> + * is going to take the lock*
>> + * taskpointer 0 lock is held (fast release possible)
>> + * taskpointer 1 lock is held and has waiters
>
> * taskpointer 1 lock is held and has waiters*
>
>
>> *
>> * The fast atomic compare exchange based acquire and release is only
>> - * possible when bit 0 and 1 of lock->owner are 0.
>> + * possible when bit 0 of lock->owner are 0.
>
> s/are/is/
>
>> *
>> - * (*) There's a small time where the owner can be NULL and the
>> - * "lock has waiters" bit is set. This can happen when grabbing the lock.
>> - * To prevent a cmpxchg of the owner releasing the lock, we need to set this
>> - * bit before looking at the lock, hence the reason this is a transitional
>> - * state.
>> + * (*) It also can be a transitional state when grabbing the lock
>> + * with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
>> + * we need to set the bit0 before looking at the lock, and the owner may be
>> + * NULL in this small time, hence this can be a transitional state.
>
>
> * (*) There is a small time when bit 0 is set but there are no
> * waiters. This can happen when grabbing the lock in the slow path.
> * To prevent a cmpxchg of the owner releasing the lock, we need to
> * set this bit before looking at the lock.
>
>

Very good! Added.

>
>> It has called try_to_take_rt_mutex() in __rt_mutex_slowlock(),
>> when timeout or got signal, it returns from __rt_mutex_slowlock()
>> with lock->wait_lock still held, and then calls remove_waiter(),
>>
>> so we don't need to call try_to_take_rt_mutex() in remove_waiter().
>> It is strange that remove_waiter() do some "require lock" work.
>
> I think you are correct here. It should never get to this path where
> !owner && first is true. If we timed out then we either got the lock or
> we could not. If we could not than owner had to be set or we are not the
> top waiter. Thus we need:
>
> if (!owner) {
> BUG_ON(first);
> return;
> }
>

The "BUG_ON" is not added, but new comments for this are added.
Because we have just tested it in try_to_take_rt_mutex(), don't need
to check it again.

If prerequisites are passed by complex paths or are
provided by different threads, I will happy to add more checks.

Thanks,
Lai