Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list is not NULL.
From: Tim Chen
Date: Thu Jan 21 2016 - 16:23:14 EST
On Thu, 2016-01-21 at 17:29 +0800, Ding Tianhong wrote:
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 0551c21..596b341 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -256,7 +256,7 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
> struct task_struct *owner;
> int retval = 1;
> - if (need_resched())
> + if (need_resched() || atomic_read(&lock->count) == -1)
> return 0;
One concern I have is this change will eliminate any optimistic spinning
as long as there is a waiter. Is there a middle ground that we
can allow only one spinner if there are waiters?
In other words, we allow spinning when
atomic_read(&lock->count) == -1 but there is no one on the
osq lock that queue up the spinners (i.e. no other process doing
This could allow a bit of spinning without starving out the waiters.