linux-kernel - Re: [PATCH] printk/nmi: Prevent deadlock when serializing NMI backtraces

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Message-ID: <20180518063820.GA1504@jagdpanzerIV>
Date:   Fri, 18 May 2018 15:38:20 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org, "4 . 13+" <stable@...r.kernel.org>
Subject: Re: [PATCH] printk/nmi: Prevent deadlock when serializing NMI
 backtraces

On (05/18/18 11:07), Sergey Senozhatsky wrote:
> 
> if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) ||
> 	raw_spin_is_locked(&logbuf_lock)
> 
> just to check per-CPU `printk_context' first and only afterwards
> access the global `logbuf_lock'. printk_nmi_enter() happens on
> every CPU, so maybe we can avoid some overhead by checking the
> local per-CPU data first.

Nah, may be it won't. This, probably, would have been the case if we
had continue to call console drivers from printk_safe section [at least].
CPUs don't spend that much time in printk_safe sections.

	-ss