linux-kernel - Re: [PATCH RFC] kexec: Freeze processes before kexec

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Date:   Thu, 17 Nov 2022 16:07:39 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Philipp Rudo <prudo@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        ribalda@...gle.com, zwisler@...gle.com, robdclark@...il.com,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec@...ts.infradead.org
Subject: Re: [PATCH RFC] kexec: Freeze processes before kexec

On Thu, Nov 17, 2022 at 3:46 PM Philipp Rudo <prudo@...hat.com> wrote:

> On Wed, 16 Nov 2022 15:16:10 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
>
> > On Wed, 16 Nov 2022 19:56:24 +0000
> > "Joel Fernandes (Google)" <joel@...lfernandes.org> wrote:
> >
> > > --- a/kernel/kexec_core.c
> > > +++ b/kernel/kexec_core.c
> > > @@ -1175,6 +1175,12 @@ int kernel_kexec(void)
> > >     } else
> > >  #endif
> > >     {
> > > +           error = freeze_processes();
> > > +           if (error) {
> > > +                   error = -EBUSY;
> > > +                   goto Unlock;
> > > +           }
> >
> > If this is the path of a kernel panic, do we really want to check the
> > return error of freeze_processes()? We are panicing, there's not much more
> > we can do.
>
> kernel_kexec isn't called during panic. We don't need to worry about it
> here.

Indeed, sorry for my hasty comment and for misleading Steve. This is
seen to happen when doing manual kexec from the reboot(2) system call.
During a regular panic, machine_shutdown() is not called so this would
not happen. However, for testing, the crash is a hurdle.

> Having that said I think this is a problem in the device driver _not_
> in kexec. In my opinion it's the job of the driver to prevent such
> races during shutdown.

Thanks a lot for your input. Rob, what do you think?

thanks,

 - Joel