Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210701125046.43018-1-wangborong@cdjrlc.com> <f72d43b9-88a1-19f0-c6ca-87fd7a01f379@csgroup.eu>
In-Reply-To: <f72d43b9-88a1-19f0-c6ca-87fd7a01f379@csgroup.eu>
From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 1 Jul 2021 15:47:48 +0200
Message-ID: <CAK8P3a2LA_MzP=mZ7_QODx5c7qq-r5t-5pZCiThjD_jX_KPXPA@mail.gmail.com>
Subject: Re: [PATCH] sched: Use WARN_ON
To:     Christophe Leroy <christophe.leroy@csgroup.eu>
Cc:     Jason Wang <wangborong@cdjrlc.com>, Jeremy Kerr <jk@ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8BIT
Precedence: bulk

On Thu, Jul 1, 2021 at 2:57 PM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
> Le 01/07/2021 à 14:50, Jason Wang a écrit :
> > The BUG_ON macro simplifies the if condition followed by BUG, but it
> > will lead to the kernel crashing. Therefore, we can try using WARN_ON
> > instead of if condition followed by BUG.
>
> But are you sure it is ok to continue if spu_acquire(ctx) returned false ?
> Shouldn't there be at least for fallback handling ?
>
> Something like:
>
>         if (WARN_ON(spu_acquire(ctx)))
>                 return;

I think you get a crash in either case:

- with the existing BUG_ON() there is an immediate backtrace and it stops there
- with WARN_ON() and continuing, you operate on a context that is not
  valid
- with the 'return', you get an endless loop, as it keeps calling
spusched_tick()
  without sleeping.

Out of those options, the existing BUG_ON() seems best.

       Arnd