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 22:13:31 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Leonardo Bras <leonardo@...ux.ibm.com>,
        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>,
        Leonardo Bras <leonardo@...ux.ibm.com>,
        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

Leonardo Bras <leonardo@...ux.ibm.com> writes:
> Currently, if printk lock (logbuf_lock) is held by other thread during
> crash, there is a chance of deadlocking the crash on next printk, and
> blocking a possibly desired kdump.
>
> After sending IPI to all other CPUs, make printk enter in NMI context,
> as it will use per-cpu buffers to store the message, and avoid locking
> logbuf_lock.
>
> Signed-off-by: Leonardo Bras <leonardo@...ux.ibm.com>
> ---
>  arch/powerpc/kexec/crash.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
> index d488311efab1..9b73e3991bf4 100644
> --- a/arch/powerpc/kexec/crash.c
> +++ b/arch/powerpc/kexec/crash.c
> @@ -115,6 +115,7 @@ static void crash_kexec_prepare_cpus(int cpu)

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.

cheers