linux-kernel - Re: [PATCH v2] printk: Use the main logbuf in NMI when logbuf

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Message-ID: <20170505022141.GA418@tigerII.localdomain>
Date:   Fri, 5 May 2017 11:21:41 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Russell King <rmk+kernel@....linux.org.uk>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chris Metcalf <cmetcalf@...hip.com>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v2] printk: Use the main logbuf in NMI when logbuf_lock
 is available

On (05/04/17 17:46), Petr Mladek wrote:
[..]
> Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Signed-off-by: Petr Mladek <pmladek@...e.com>

Acked-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

a small nitpick,

[..]
>  void printk_nmi_enter(void)
>  {
> -	this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> +	/*
> +	 * The size of the extra per-CPU buffer is limited. Use it only when
> +	 * the main one is locked. If this CPU is not in the safe context,
> +	 * the lock must be taken on another CPU and we could wait for it.
> +	 */
> +	if (raw_spin_is_locked(&logbuf_lock) &&
> +	    this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) {
> +		this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);

could we please check `printk_context' (local to a particular CPU)
first and, if positive, then access `logbuf_lock' (which is global)?

	-ss