linux-kernel - Re: [PATCH] printk: drop in_nmi check from printk_safe_flush_on

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Date:   Thu, 31 May 2018 16:21:00 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] printk: drop in_nmi check from
 printk_safe_flush_on_panic()

On Wed 2018-05-30 19:00:37, Sergey Senozhatsky wrote:
> On (05/30/18 18:55), Sergey Senozhatsky wrote:
> > The thing is, we, in fact, already invoke panic() in printk_safe mode.
> > Sometimes.
> > 
> > Namely,
> > 
> >   nmi_panic() -> panic()
> > 
> > is invoked while we are in printk_nmi(), so all printk()-s go
> > to the per-CPU buffers. So, at least to some extent, panic()
> > in printk_safe context is not something never seen before. Just
> > saying.
> 
> Well, we have a PRINTK_NMI_DEFERRED_CONTEXT_MASK mode for
> printk_nmi(). May be we can [if need be] come up with the same trick
> for printk_safe_panic() mode. If logbuf spin_lock is unlocked, then
> we use the main logbuf, if it is locked, we redirect printk to per-CPU
> buffers and then flush it via printk_safe_flush_on_panic(), which will
> re-init (unlock) the logbuf.

All these checks are racy. Now, I believe that it might really prevent
a deadlock in some situations but it might also cause loosing messages
in other situations (never flushed buffer). I am sorry but I am still
unable to decide if it is worth the risk.

I would want to keep it as is until anyone comes with a more detailed
analyze or until we get some bug reports.

Best Regards,
Petr