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

[Bart Van Assche]

On Tue, Apr 29, 2008 at 4:57 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> 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@xxxxxxxxxx> 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.
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