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

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Date:	Tue, 02 Mar 2010 22:43:20 +0900
From:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
To:	Peter Zijlstra <peterz@...radead.org>
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 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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