linux-kernel - Re: [RFC][PATCH 0/7] lockdep: Support recurise-read locks

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Message-ID: <BANLkTikXSz-AK6BFvnDGEr6Qhx-dekzOQg@mail.gmail.com>
Date:	Fri, 22 Apr 2011 16:01:05 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	a.p.zijlstra@...llo.nl, rostedt@...dmis.org, tglx@...utronix.de,
	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/7] lockdep: Support recurise-read locks

2011/4/22 Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>:
> Yong Zhang wrote:
>> > Also, something is still wrong because lockdep fails to detect the problem
>> > for "cat /proc/locktest1 /proc/locktest2" and
>> > "cat /proc/locktest3 /proc/locktest4" cases.
>>
>> It fails because we never add the recursive read to prev->after evev if
>> it passed the validation.
>>
> Thanks. But why "cat /proc/locktest1 /proc/locktest2" is "the recursive read"
> and "cat /proc/locktest2 /proc/locktest1" is not "the recursive read"?
> Both are serialized. Both hold and release the same lock.
> The only difference is which function was called first,

When you are using rwlock_acquire*(), your four testcases are
all failed, the reason I have explained.

When you are using spin_acquire()/spin_release() in read_seqbegin2()/
read_seqretry2(), if you call locktest2/locktest4 firstly, the chain will
be established, so when call locktest1/locktest3, lockdep warns on it.
But if you call locktest1/locktest2 firstly, the chain will not be established
just because recursive read is not added to prev->after.

> and lockdep alart depends on which function was called first.

No, it's not related with which function is called firstly, instead it's
the current behavior of lockdep.

>
> It sounds to me that Documentation/lockdep-design.txt says
> timing (i.e. which function was called first) is not important.
>
> 172 Proof of 100% correctness:
> 173 --------------------------
> 174
> 175 The validator achieves perfect, mathematical 'closure' (proof of locking
> 176 correctness) in the sense that for every simple, standalone single-task
> 177 locking sequence that occurred at least once during the lifetime of the
> 178 kernel, the validator proves it with a 100% certainty that no
> 179 combination and timing of these locking sequences can cause any class of
> 180 lock related deadlock. [*]
> 181
> 182 I.e. complex multi-CPU and multi-task locking scenarios do not have to
> 183 occur in practice to prove a deadlock: only the simple 'component'
> 184 locking chains have to occur at least once (anytime, in any
> 185 task/context) for the validator to be able to prove correctness. (For
> 186 example, complex deadlocks that would normally need more than 3 CPUs and
> 187 a very unlikely constellation of tasks, irq-contexts and timings to
> 188 occur, can be detected on a plain, lightly loaded single-CPU system as
> 189 well!)

This is true, but currently we take different action on recursive read
validation
which seems needed to be improved. :)

Thanks,
Yong



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