Re: [RFC][PATCH -v2 4/4] locking/mutex: Add lock handoff to avoid starvation

[Waiman Long <waiman.long@hpe.com>]

On 08/25/2016 02:37 PM, Peter Zijlstra wrote:
> @@ -468,9 +496,12 @@ void __sched mutex_unlock(struct mutex *
>   	DEBUG_LOCKS_WARN_ON(__mutex_owner(lock) != current);
>   #endif
>
> -	owner = atomic_long_fetch_and(MUTEX_FLAGS,&lock->owner);
> +	owner = atomic_long_read(&lock->owner);
> +	if (!(owner&  MUTEX_FLAG_HANDOFF))
> +		owner = atomic_long_fetch_and(MUTEX_FLAGS,&lock->owner);
> +
>   	if (__owner_flags(owner))
> -		__mutex_unlock_slowpath(lock);
> +		__mutex_unlock_slowpath(lock, owner);
>   }
>   EXPORT_SYMBOL(mutex_unlock);

I don't think the race condition is fixed when we don't make sure that 
lock handoff only happens from current=>new. The problem is due to the 
fact that the MUTEX_FLAG_HANDOFF check in the unlock fastpath isn't 
serialized by the wait_lock. As a result, it is possible that the owner 
is NULL while the HANDOFF bit is set. Or an optimistic spinner may have 
stolen the lock in the interim.

Cheers,
Longman