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

[Bart Van Assche]

On Tue, Apr 29, 2008 at 5:15 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> 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@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).
>
>  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.
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