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

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Date:   Wed, 11 Oct 2017 11:36:36 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Fengguang Wu <fengguang.wu@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.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:
> On Tue, Oct 10, 2017 at 9:22 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I really would like to see the sites that do cross-thread lock/unlock
> > pairs themselves be annotated.
> >
> > So when you lock in one thread, and then unlock in another, I'd
> > actually prefer to see something like
> >
> >  - T1:
> >         lock_mutex_cross();
> >
> >  - T2:
> >         unlock_mutex_cross();
> >
> > to make it very explicit that *these* particular lock/unlock
> > operations are the fancy ones.
> 
> Actually, let's make it even *more* obvious, and even easier for
> lockdep (and for humans) to see what's going on.
> 
> 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);

In addition, if we really need to use mutex locks in this way, I also
think it should work with the explicit primitives as you said.