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Date:   Fri, 2 Oct 2020 20:36:02 +0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     g@hirez.programming.kicks-ass.net, Qian Cai <cai@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: lockdep null-ptr-deref
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On Wed, Sep 30, 2020 at 09:02:28PM +0200, Peter Zijlstra wrote:
> 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 ?
> 

But what if f2() is called with interrupt disabled? Or f2() disables
interrupt inside the function, like:

	void f2(...)
	{
		local_irq_disable();
		spin_lock(&B);
		g(...);
		...
		local_irq_enable();
	}

In this case, there wouldn't be any LOCK_ENABLED_*_READ usage for
rwlock_t A. As a result, we won't see it in the lockdep splat.

Regards,
Boqun

> 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.