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SubjectRe: [PATCH] spinlock: __raw_spin_is_locked() should return true for UP
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On Wed, 2009-08-19 at 11:31 +0200, Olivier Galibert wrote:
> On Tue, Aug 18, 2009 at 07:40:16PM -0700, Linus Torvalds wrote:
> >
> >
> > On Tue, 18 Aug 2009, Kumar Gala wrote:
> > >
> > > I agree its a little too easy to abuse spin_is_locked. However we should be
> > > consistent between spin_is_locked on UP between with and without
> > > CONFIG_DEBUG_SPINLOCK enabled.
> >
> > No we shouldn't.
> >
> > With CONFIG_DEBUG_SPINLOCK, you have an actual lock variable for debugging
> > purposes, so spin_is_locked() can clearly return a _valid_ answer, and
> > should do so.
> >
> > Without DEBUG_SPINLOCK, there isn't any answer to return.
> >
> > So there's no way we can or should be consistent. In one case an answer
> > exists, in another one the answer is meaningless and doesn't exist.
>
> I always thought behaviour should be consistent between code with
> debugging on and code without. Otherwise you may end up with cases of
> "it starts working when I turn on debugging" which are a pain to fix.
> Has something changed?
>
> Or in other words, do you think lockdep should try solving deadlocks
> instead of just reporting them for instance?

The point is spin_is_locked() is a broken interface in that respect. Its
plain impossible to give the right answer.


Suppose there's code doing:

/*
* Ensure we don't have foo lock taken, because that would cause
* lock inversion under bar lock.
*/
BUG_ON(spin_is_locked(&foo));
spin_lock(&bar);
and other code doing:

/*
* Ensure we've got foo locked because it protects bar
*/
BUG_ON(!spin_is_locked(&foo));
bar = fancy;
What value should you return when locks don't exist (which is the case
for UP)?


There simply is no right answer other than: don't use spin_is_locked().


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