Re: [patch 2/2] x86_64: ticket lock spinlock

Nick Piggin
2007-08-09T01:40:30Z

On Wed, Aug 08, 2007 at 01:31:58PM -0400, Valdis.Kletnieks@vt.edu wrote:
> On Wed, 08 Aug 2007 06:24:44 +0200, Nick Piggin said:
>
> > After this, we can no longer spin on any locks with preempt enabled,
> > and cannot reenable interrupts when spinning on an irq safe lock, because
> > at that point we have already taken a ticket and the would deadlock if
> > the same CPU tries to take the lock again. These are hackish anyway: if
> > the lock happens to be called under a preempt or interrupt disabled section,
> > then it will just have the same latency problems. The real fix is to keep
> > critical sections short, and ensure locks are reasonably fair (which this
> > patch does).
>
> Any guesstimates how often we do that sort of hackish thing currently, and
> how hard it will be to debug each one? "Deadlock if the same CPU tries to
> take the lock again" is pretty easy to notice - are there more subtle failure
> modes (larger loops of locks, etc)?

I'll try to explain better:

The old spinlocks re-enable preemption and interrupts while they spin
waiting for a held lock. This was done because people noticed some
long latencies while spinning. The problem however is that preemption
and interrupts can only be re-enabled if they were enabled before the
spin_lock call. So if you have code that perhaps takes nested locks,
or locks while interrupts are already disabled, then you get the latency
problems back.

So the non-hack fix is to keep critical sections short (which is what
we've been working at forever), and to have relatively fair locks
(which is what this patch does).

A side-effect of this patch is that it can no longer enable preemption
or ints while spinning, so my changelog is a rationale of why that
shouldn't be a big problem.