Messages in this thread | | | Date | Sun, 2 Feb 2014 22:12:30 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH v2 5/5] mutex: Give spinners a chance to spin_on_owner if need_resched() triggered while queued |
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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.
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