Re: [patch] Real-Time Preemption, -RT-2.6.10-rc1-mm2-V0.7.1

[Ingo Molnar <mingo@elte.hu>]

* Scott Wood <scott@timesys.com> wrote:

> > is the order of locks in the dependency chain really unpredictable? If
> > two chain walkers get two locks in opposite order, doesnt that mean that
> > the lock ordering (as attempted by the blocked tasks) is deadlock-prone
> > already? I.e. this scenario should not happen.
> 
> It *shouldn't*, but bugs do happen, and it'd be nice if a mutex
> deadlock didn't get promoted into a less debuggable spinlock deadlock.
> [...]

well, deadlock detection happens at lock-acquire time, so the deadlock
will be detected _first_, any PI spinlock-locking will happen on already
blocked (== no deadlock detected) tasks. This would also serve as a nice
secondary check for the deadlock detector.

> [...] Plus, if there's any intention of ever exporting this priority
> inheritance mechanism to userspace locks, we don't want to promote a
> userspace deadlock into a kernel one.

agreed.

> Given how rarely contention should occur, I don't think that a single
> lock would be a bottleneck except for obscenely large SMP machines.

well, blocking on a mutex happens quite frequently. But i dont have a
problem with the big lock other than the usual "if we can do better then
we should do better" attitude :-)

	Ingo