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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
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