Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

From: Qian Cai
Date: Tue Oct 08 2019 - 09:23:57 EST

On Tue, 2019-10-08 at 09:13 -0400, Steven Rostedt wrote:
> On Tue, 8 Oct 2019 10:15:10 +0200
> Petr Mladek <pmladek@xxxxxxxx> wrote:
> > There are basically three possibilities:
> >
> > 1. Do crazy exercises with locks all around the kernel to
> > avoid the deadlocks. It is usually not worth it. And
> > it is a "whack a mole" approach.
> >
> > 2. Use printk_deferred() in problematic code paths. It is
> > a "whack a mole" approach as well. And we would end up
> > with printk_deferred() used almost everywhere.
> >
> > 3. Always deffer the console handling in printk(). This would
> > help also to avoid soft lockups. Several people pushed
> > against this last few years because it might reduce
> > the chance to see the message in case of system crash.
> >
> > As I said, there has finally been agreement to always do
> > the offload few weeks ago. John Ogness is working on it.
> > So we might have the systematic solution for these deadlocks
> > rather sooner than later.
> Another solution is to add the printk_deferred() in these places that
> cause lockdep splats, and when John's work is done, it would be easy to
> grep for them and remove them as they would no longer be needed.
> This way we don't play whack-a-mole forever (only until we have a
> proper solution) and everyone is happy that we no longer have these
> false positive or I-don't-care lockdep splats which hide real lockdep
> splats because lockdep shuts off as soon as it discovers its first
> splat.

I feel like that is what I trying to do, but there seems a lot of resistances
with that approach where pragmatism met with perfectionism.