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

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Message-ID: <20180530100037.GE12230@jagdpanzerIV>
Date:   Wed, 30 May 2018 19:00:37 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH] printk: drop in_nmi check from
 printk_safe_flush_on_panic()

On (05/30/18 18:55), Sergey Senozhatsky wrote:
> > 
> > I understand why you came with it but I am against this change without
> > a proper research. This would redirect too valuable messages into
> > a buffer of a limited size and postpone flushing them to the consoles.
> > 
> > We would need to really carefully compare chances where this would
> > help and where it would make things worse. There is a high chance
> > that we could come with a better solution once we have the analyze.
> 
> I agree, sure.
> 
> 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.

	-ss