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Message-ID: <1452b1d770bd6c516606e4855f348773d1fa6e59.camel@linux.ibm.com>
Subject: Re: [PATCH 1/1] powerpc/crash: Use NMI context for printk after
 crashing other CPUs
From:   Leonardo Bras <leonardo@linux.ibm.com>
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Enrico Weigelt <info@metux.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Allison Randal <allison@lohutok.net>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Nicholas Piggin <npiggin@gmail.com>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Date:   Wed, 08 Apr 2020 15:41:37 -0300
In-Reply-To: <87y2r6jhuc.fsf@mpe.ellerman.id.au>
References: <20200406174058.686436-1-leonardo@linux.ibm.com>
         <87y2r6jhuc.fsf@mpe.ellerman.id.au>
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On Wed, 2020-04-08 at 22:13 +1000, Michael Ellerman wrote:
[...]
> Added context:
>=20
> 	printk(KERN_EMERG "Sending IPI to other CPUs\n");
>=20
> 	if (crash_wake_offline)
> 		ncpus =3D num_present_cpus() - 1;
>=20
> > =20
> >  	crash_send_ipi(crash_ipi_callback);
> >  	smp_wmb();
> > +	printk_nmi_enter();
>  =20
> Why did you decide to put it there, rather than at the start of
> default_machine_crash_shutdown() like I did?
>=20
> 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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