linux-kernel - Re: [PATCH 1/1] powerpc/crash: Use NMI context for printk after crashing other CPUs

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Date:   Wed, 08 Apr 2020 15:41:37 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Enrico Weigelt <info@...ux.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Allison Randal <allison@...utok.net>,
        Christophe Leroy <christophe.leroy@....fr>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] powerpc/crash: Use NMI context for printk after
 crashing other CPUs

On Wed, 2020-04-08 at 22:13 +1000, Michael Ellerman wrote:
[...]
> Added context:
> 
> 	printk(KERN_EMERG "Sending IPI to other CPUs\n");
> 
> 	if (crash_wake_offline)
> 		ncpus = num_present_cpus() - 1;
> 
> >  
> >  	crash_send_ipi(crash_ipi_callback);
> >  	smp_wmb();
> > +	printk_nmi_enter();
>   
> Why did you decide to put it there, rather than at the start of
> default_machine_crash_shutdown() like I did?
> 
> The printk() above could have already deadlocked if another CPU is stuck
> with the logbuf lock held.

Oh, I thought the CPUs would start crashing after crash_send_ipi(), so
only printk() after that would possibly deadlock.

I was not able to see how the printk() above would deadlock, but I see
no problem adding that at the start of the function.

Best regards,
Leonardo Bras

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