Messages in this thread | | | Date | Wed, 19 Feb 2014 14:30:31 -0500 | From | Waiman Long <> | Subject | Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation |
| |
On 02/19/2014 03:55 AM, Peter Zijlstra wrote: > On Tue, Feb 18, 2014 at 07:58:49PM -0500, Waiman Long wrote: >> On 02/18/2014 04:37 PM, Peter Zijlstra wrote: >>> On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote: >>>>>> + /* >>>>>> + * At the head of the wait queue now >>>>>> + */ >>>>>> + while (true) { >>>>>> + u32 qcode; >>>>>> + int retval; >>>>>> + >>>>>> + retval = queue_get_lock_qcode(lock,&qcode, my_qcode); >>>>>> + if (retval> 0) >>>>>> + ; /* Lock not available yet */ >>>>>> + else if (retval< 0) >>>>>> + /* Lock taken, can release the node& return */ >>>>>> + goto release_node; >>>>>> + else if (qcode != my_qcode) { >>>>>> + /* >>>>>> + * Just get the lock with other spinners waiting >>>>>> + * in the queue. >>>>>> + */ >>>>>> + if (queue_spin_trylock_unfair(lock)) >>>>>> + goto notify_next; >>>>> Why is this an option at all? >>>>> >>>>> >>>> Are you referring to the case (qcode != my_qcode)? This condition will be >>>> true if more than one tasks have queued up. >>> But in no case should we revert to unfair spinning or stealing. We >>> should always respect the queueing order. >>> >>> If the lock tail no longer points to us, then there's further waiters >>> and we should wait for ->next and unlock it -- after we've taken the >>> lock. >>> >> A task will be in this loop when it is already the head of a queue and is >> entitled to take the lock. The condition (qcode != my_qcode) is to decide >> whether it should just take the lock or take the lock& clear the code >> simultaneously. I am a bit cautious to use queue_spin_trylock_unfair() as >> there is a possibility that a CPU may run out of the queue node and need to >> do unfair busy spinning. > No; there is no such possibility. Add BUG_ON(idx>=4) and make sure of > it.
Yes, I could do that.
However in the generic implementation, I still need some kind of atomic cmpxchg to set the lock bit. I could probably just do a simple assignment of 1 to the lock byte in x86.
> There's simply no more than 4 contexts what can nest at any one time: > > task context > softirq context > hardirq context > nmi context > > And someone contending a spinlock from NMI context should be shot > anyway. > > Getting more nested spinlocks is an absolute hard fail.
| |