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

Bart Van Assche
2008-04-29T16:03:39Z

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.

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.

Bart.