linux-kernel - Re: Question about lock sequence

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Date:	Sat, 10 Apr 2010 16:01:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: Question about lock sequence

On Sat, 2010-04-10 at 19:44 +0900, Hitoshi Mitake wrote:
> Hi,
> 
> I found that my understand about lockdep is completely wrong :( ,
> so state machine of perf lock should be fixed before optimization.
> 
> And I found that behaviour related to some of spin locks are strange.
> The concrete example is lock sequences targeting dcache_lock (defined in
> head of fs/dcache.c).
> 
> I made a little (and not essential) change to perf lock, and observe
> lock sequence targeting it.
> Changed perf lock shows sequence of locks in time order,
> and I grepped the output of it with dcache, like this:
> 
> % sudo ./perf lock report | grep dcache
> 
> The head part of result is this:
> # <name>-<pid> <time (in u64)> <action> <address of lockdep> <name of lock>
> perf-3238 92430534170 acquire: 0xffffffff81a4b398 dcache_lock
> perf-3238 92430536714 acquire: 0xffffffff81a4b398 dcache_lock
> perf-3238 92431444481 acquire: 0xffffffff81a4b398 dcache_lock
> perf-3238 92431446061 acquired: 0xffffffff81a4b398 dcache_lock
> perf-3238 92431448157 acquire: 0xffffffff81a4b398 dcache_lock
> perf-3238 92431449670 acquired: 0xffffffff81a4b398 dcache_lock
> perf-3238 92432371136 acquire: 0xffffffff81a4b398 dcache_lock
> perf-3238 92432372712 acquired: 0xffffffff81a4b398 dcache_lock
> perf-3238 92432374718 acquire: 0xffffffff81a4b398 dcache_lock
> perf-3238 92432376173 acquired: 0xffffffff81a4b398 dcache_lock
> perf-3238 92433315563 acquire: 0xffffffff81a4b398 dcache_lock
> perf-3238 92433317173 acquired: 0xffffffff81a4b398 dcache_lock
> 
> There are too many acquire and acquired without corresponding release
> (or contended).
> If dcache_lock is rwlock and these acquires mean read locks, this is not
> so strange.
> But, for me, this is a pattern of dead lock.
> Of course perf lock finished its work, so there is no actual dead lock.
> 
> If you know something about this behaviour of lock, could you tell me?

Well dcache_lock is a regular spinlock and there is only one of them, my
guess is that your timeline got messed up somehow.

Also, there doesn't appear to be a proper balance between acquires and
releases.


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