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

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Date:   Wed, 23 May 2018 11:01:07 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, "4 . 13+" <stable@...r.kernel.org>
Subject: Re: [PATCH] printk/nmi: Prevent deadlock when serializing NMI
 backtraces

On (05/22/18 17:43), Steven Rostedt wrote:
> > CPU0			CPU1			CPU2
> > 
> > printk()
> >   vprintk_emit()
> >     spin_lock(&logbuf_lock)
> > 
> > 						trigger_all_cpu_backtrace()
> > 						  raise()
> > 
> > 			nmi_enter()
> > 			  printk_nmi_enter()
> > 			    if (this_cpu_read(printk_context)
> > 			      & PRINTK_SAFE_CONTEXT_MASK)
> > 			      // false
> > 			    else
> > 			      // looks safe to use printk_deferred()
> > 			      this_cpu_or(printk_context,
> > 				PRINTK_NMI_DEFERRED_CONTEXT_MASK);
> > 
> > 			  nmi_cpu_backtrace()
> > 			    arch_spin_lock(&lock);
> 
> What branch is this based on, because I can't find the
> "arch_spin_lock()" you are talking about here.

This arch_spin_lock() is in lib/nmi_backtrace.c

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/nmi_backtrace.c?h=v4.17-rc6#n94

	-ss