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

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Date:   Wed, 11 Oct 2017 10:14:58 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 08:14:09PM +0200, Peter Zijlstra wrote:
> 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.

Plus, lock_page(). Honestly, I want that to be the main beneficiary when
we talking about crossrelease.

Actually, I started the crossrelease work to detect deadlocks by
lock_page() and expect it's more useful.