Re: Question about policy of calling lockdep functions in trylocks

[Hitoshi Mitake]

On 03/02/10 18:13, Peter Zijlstra wrote:
> On Tue, 2010-03-02 at 17:44 +0900, Hitoshi Mitake wrote:
> > Hi,
> >
> > I have a question about policy of callings lockdep functions in trylocks.
> >
> > Normal locks like __raw_spin_lock are defined like this:
> >
> > static inline void __raw_spin_lock(raw_spinlock_t *lock)
> > {
> > 	preempt_disable();
> > 	spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
> > 	LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
> > }
> >
> > And LOCK_CONTENDED is defined:
> > #define LOCK_CONTENDED(_lock, try, lock)			\
> > do {								\
> > 	if (!try(_lock)) {					\
> > 		lock_contended(&(_lock)->dep_map, _RET_IP_);	\
> > 		lock(_lock);					\
> > 	}							\
> > 	lock_acquired(&(_lock)->dep_map, _RET_IP_);			\
> > } while (0)
> >
> > So, acquiring and releasing lock with no contention calls lockdep
> > functions like this:
> >
> >       lock_acquire ->  lock_acquired ->  lock_release
> >
> > And acquiring and releasing lock with contention calls lockdep functions
> > like this:
> >
> >       lock_acquire ->  lock_contended ->  lock_acquired ->  lock_release
> >
> > But I found that locks with try like __raw_spin_trylock is defined like
> > this:
> >
> > static inline int __raw_spin_trylock(raw_spinlock_t *lock)
> > {
> > 	preempt_disable();
> > 	if (do_raw_spin_trylock(lock)) {
> > 		spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);
> > 		return 1;
> > 	}
> > 	preempt_enable();
> > 	return 0;
> > }
> >
> > So, trying acquiring and releasing lock with no contention calls lockdep
> > functions like this:
> >
> >       lock_acquire ->  lock_release
> >
> > And failed trying acquiring calls no lockdep function.
> >
> > I felt that policy of calling lockdep functions is strange.
> > Trylocks should be like this:
> >
> > static inline int __raw_spin_trylock(raw_spinlock_t *lock)
> > {
> > 	preempt_disable();
> > 	spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);
> > 	if (do_raw_spin_trylock(lock)) {
> > 		spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);
> > 		lock_acquired(&lock->dep_map, _RET_IP_);
> > 		return 1;
> > 	}
> > 	lock_contended(&lock->dep_map, _RET_IP_);
> > 	preempt_enable();
> > 	return 0;
> > }
>
> Did you mean to call acquire twice?

Sorry, this is my mistake.
I've forgot to remove spin_acquire() after do_raw_spin_trylock().

> > This is my question.
> > Are there some reasons current calling lockdep functions of trylocks?
> > If not, can I change these trylocks like I described above?
> >
> > The reason why I'm asking about it is perf lock.
> > For state machine of perf lock, these event sequenses are very confusable.
> > Because sequence of trylock is subset of normal lock. This is ambiguity.
>
> Well, trylocks cannot contend, so the lock_contended() call doesn't make
> sense, I don't think it will confuse lockstat, since acquire will simply
> reset the state again, but it will waste cycles, nor is there a reason
> to call acquired without first having call contended. So no.
>
> What exactly is the problem, the lack of callbacks for a failed trylock?
> Why would you want one?
>
> Because other than that I see no problem, you get an acquire(.try=1) and
> a release() to match and lockstat measure and accounts the hold-time.

Ah, I've forgot about try and read parameters of lock_acquire().
These parameters are enough things for me.
perf lock can separate event sequences of normal locks and trylocks with 
these.

Thanks!
	Hitoshi


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