Re: [PATCH 1/8] lockdep: fix recursive read lock validation

Peter Zijlstra
2008-04-29T16:15:57Z

On Tue, 2008-04-29 at 18:03 +0200, Bart Van Assche wrote:
> On Tue, Apr 29, 2008 at 5:15 PM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> >
> > On Tue, 2008-04-29 at 17:03 +0200, Bart Van Assche wrote:
> > > On Tue, Apr 29, 2008 at 4:57 PM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > > >
> > > > On Tue, 2008-04-29 at 15:16 +0200, Bart Van Assche wrote:
> > > > > On Tue, Apr 29, 2008 at 2:57 PM, Gautham R Shenoy <ego@in.ibm.com> wrote:
> > > > > > Subject: lockdep: fix recursive read lock validation
> > > > > > This means that the following sequence is now invalid, whereas previously
> > > > > > it was considered valid:
> > > > > >
> > > > > > rlock(a); rlock(b); runlock(b); runlock(a)
> > > > > > rlock(b); rlock(a);
> > > > >
> > > > > Why are you marking this sequence as invalid ? Although it can be
> > > > > debated whether it is good programming practice to be inconsistent
> > > > > about the order of read-locking, the above sequence can't be involved
> > > > > in a deadlock.
> > > >
> > > > Not for pure read locks, but when you add write locks to it, it does get
> > > > deadlocky. Lockdep does not keep separate chains for read and write
> > > > locks.
> > >
> > > Nesting writer locks inside reader locks is always a bad idea. So
> > > please come up with an example of how varying the reader lock nesting
> > > order can trigger a deadlock (when no writer locks are nested inside
> > > reader locks and nested writer locks are always nested in the same
> > > order).
> >
> > It can't deadlock when only readers are involved, but lockdep will not
> > be able to distinguish between the cases where only read locks are
> > involved and a mix of readers and writers is involved.
> >
> > Hence disallow both.
> >
> > But hitting this requires you do a series of rather unfortunate things:
> >
> > 1) use recursive locking
> > 2) don't have strict lock order
> > 3) make it work by using read locks only
> >
> > Seriously, any code that triggers this might want to have its locking
> > re-throught.
>
> You did not get my point.
>
> My point is that if you follow the following locking discipline, a
> deadlock will never be triggered:
> * Always obtain writer locks in a consistent order.
> * Never nest writer locks inside reader locks.
> * Nesting reader locks inside writer locks is okay, and nesting reader
> locks inside other reader locks is also OK.
>
> Again: if you do not agree with the above, please post an example that
> proves me wrong.

Using a lock that does not allow reader nesting would be cheating,
right?

> Or: whether or not to allow a sequence like "rlock(a); rlock(b);
> runlock(b); runlock(a); rlock(b); rlock(a);" is something we can
> choose. We do not have to forbid this sequence -- we can choose
> whether or not we allow this sequence.

I'm utterly confused now; I never argued that it would get deadlocks;
and I said I choose to not allow it from a lockdep pov. What else do you
want?