linux-kernel - Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer dereference at 000001f2

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Date:   Tue, 10 Oct 2017 20:14:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Byungchul Park <byungchul.park@....com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        LKP <lkp@...org>, Josh Poimboeuf <jpoimboe@...hat.com>,
        kernel-team@....com
Subject: Re: [lockdep] b09be676e0 BUG: unable to handle kernel NULL pointer
 dereference at 000001f2

On Tue, Oct 10, 2017 at 09:56:26AM -0700, Linus Torvalds wrote:

> So I think the best model would be something like this:
> 
>  - T1:
>         mutex_lock(&lock)
>         ...
>         mutex_transfer(&lock)
> 
>  - T2:
>         mutex_receive(&lock);
>         ...
>         mutex_unlock(&lock);
> 
> where the "mutex_transfer() -> mutex_receive()" thing really makes it
> obvious that "now thread 1 is transferring the lock to thread 2".

Ah, but that's not at all what cross-release is about. Nobody really
does wonky ownership transfer of mutexes like that (although there might
be someone doing something with semaphores, I didn't check). Its to
allow detecting this deadlock:

	mutex_lock(&lock)
	wait_for_completion(&c);
					mutex_lock(&lock);
					complete(&c);

The completion doesn't have an owner to transfer.