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

Bart Van Assche
2008-04-29T15:03:49Z

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).

Bart.