Messages in this thread | | | Date | Thu, 31 Mar 2016 16:39:59 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v3 3/3] locking/mutex: Avoid missed wakeup of mutex waiter |
| |
On 03/29/2016 12:36 PM, Peter Zijlstra wrote: > On Tue, Mar 22, 2016 at 01:46:44PM -0400, Waiman Long wrote: >> The current mutex code sets count to -1 and then sets the task >> state. This is the same sequence that the mutex unlock path is checking >> count and task state. That could lead to a missed wakeup even though >> the problem will be cleared when a new waiter enters the waiting queue. >> >> This patch reverses the order in the locking slowpath so that the task >> state is set first before setting the count. This should eliminate >> the potential missed wakeup and improve latency. > Is it really a problem though? > > So the 'race' is __mutex_lock_common() against > __mutex_fastpath_unlock(), and that is fully serialized as per the > atomic instructions. Either the fast unlock path does 1->0 and the lock > acquires, or the lock sets -1, at which the unlock fails and enters > __mutex_unlock_common_slowpath, which is fully serialised against > __mutex_lock_common by the lock->wait_lock. > > I agree that the code is nicer after your patch, but I don't actually > see a problem.
You are right again. I think I missed the spinlock serialization part from my analysis. So this patch isn't really necessary. I can withdraw it or mark it as a cleanup.
Cheers, Longman
| |