linux-kernel - Re: lockdep null-ptr-deref

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Date:   Wed, 30 Sep 2020 21:02:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     g@...ez.programming.kicks-ass.net, Qian Cai <cai@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: lockdep null-ptr-deref

On Wed, Sep 30, 2020 at 08:18:18PM +0800, Boqun Feng wrote:

> For one thing, I do think that LOCK_READ_USED trace is helpful for
> better reporting, because if there is a read lock in the dependency path
> which causes the deadlock, it's better to have the LOCK_READ_USED trace
> to know at least the initial READ usage. For example, if we have
> 
> 	void f1(...)
> 	{
> 		write_lock(&A);
> 		spin_lock(&C);
> 		// A -> C
> 		...
> 	}
> 
> 	void g(...)
> 	{
> 		read_lock(&A);
> 		...
> 	}
> 	void f2(...)
> 	{
> 		spin_lock(&B);
> 		g(...);
> 		// B -> A
> 	}
> 
> 	void f3(...) {
> 		spin_lock(&C);
> 		spin_lock(&B);
> 		// C -> B, trigger lockdep splat
> 	}
> 
> when lockdep reports the deadlock (at the time f3() is called), it will
> be useful if we have a trace like:
> 
> 	INITIAL READ usage at:
> 	g+0x.../0x...
> 	f2+0x.../0x...
> 
> Thoughts?

Wouldn't that also be in LOCK_ENABLED_*_READ ?

That is, with PROVE_LOCKING on, the initial usage is bound to set more
states, except for !check||trylock usage, and those aren't really all
that interesting.