Re: [RFC][PATCH v2 5/5] mutex: Give spinners a chance to spin_on_owner if need_resched() triggered while queued

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Feb 02, 2014 at 01:01:23PM -0800, Jason Low wrote:
> On Fri, 2014-01-31 at 21:08 +0100, Peter Zijlstra wrote:
> > On Fri, Jan 31, 2014 at 12:01:37PM -0800, Jason Low wrote:
> > > Currently still getting soft lockups with the updated version.
> > 
> > Bugger.. ok clearly I need to think harder still. I'm fairly sure this
> > cancelation can work though, just seems tricky to get right :-)
> 
> Ok, I believe I have found a race condition between m_spin_lock() and
> m_spin_unlock().
> 
> In m_spin_unlock(), we do "next = ACCESS_ONCE(node->next)". Then, if
> next is not NULL, we proceed to set next->locked to 1.
> 
> A thread in m_spin_lock() in the unqueue path could execute
> "next = cmpxchg(&prev->next, node, NULL)" after the thread in
> m_spin_unlock() accesses its node->next and finds that it is not NULL.
> Then, the thread in m_spin_lock() could check !node->locked before
> the thread in m_spin_unlock() sets next->locked to 1.

Yes indeed. How silly of me to not spot that!

> The following addition change was able to solve the initial lockups that were
> occurring when running fserver on a 2 socket box.
> 
> ---
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 9eb4dbe..e71a84a 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -513,8 +513,13 @@ static void m_spin_unlock(struct m_spinlock **lock)
>  			return;
>  
>  		next = ACCESS_ONCE(node->next);
> -		if (unlikely(next))
> -			break;
> +
> +		if (unlikely(next)) {
> +			next = cmpxchg(&node->next, next, NULL);
> +
> +			if (next)

The cmpxchg could fail and next still be !NULL I suppose.

> +				break;
> +		}


The way I wrote that same loop in step-B, is:


	for (;;) {
		if (*lock == node && cmpxchg(lock, node, prev) == node)
			return

		next = xchg(&node->next, NULL); /* B -> A */
		if (next)
			break;

		arch_mutex_cpu_relax();
	}

I suppose we can make that something like:


	if (node->next) {
		next = xchg(&node->next, NULL);
		if (next)
			break
	}

To avoid the xchg on every loop.

I had wanted to avoid the additional locked op in the unlock path, but
yes that does make things easier.