linux-kernel - Re: Question about policy of calling lockdep functions in trylocks

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Date:	Tue, 02 Mar 2010 10:13:31 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question about policy of calling lockdep functions in trylocks

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?

> 
> 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.





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