linux-kernel - Re: [PATCH v2 1/5] printk/nmi: Generic solution for safe printk in NMI

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Date:	Wed, 02 Dec 2015 13:45:16 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Petr Mladek <pmladek@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mips@...ux-mips.org,
	Daniel Thompson <daniel.thompson@...aro.org>,
	Jiri Kosina <jkosina@...e.com>, linux-cris-kernel@...s.com,
	linux-s390@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	adi-buildroot-devel@...ts.sourceforge.net,
	Ingo Molnar <mingo@...hat.com>, linux-sh@...r.kernel.org,
	sparclinux@...r.kernel.org,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/5] printk/nmi: Generic solution for safe printk in
 NMI

On Fri, 2015-11-27 at 12:09 +0100, Petr Mladek wrote:

> printk() takes some locks and could not be used a safe way in NMI
> context.
> 
> The chance of a deadlock is real especially when printing
> stacks from all CPUs. This particular problem has been addressed
> on x86 by the commit a9edc8809328 ("x86/nmi: Perform a safe NMI stack
> trace on all CPUs").

...

> diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
> new file mode 100644
> index 000000000000..3989e13a0021
> --- /dev/null
> +++ b/kernel/printk/nmi.c
> @@ -0,0 +1,200 @@

...

> +
> +struct nmi_seq_buf {
> +	atomic_t		len;	/* length of written data */
> +	struct irq_work		work;	/* IRQ work that flushes the buffer */
> +	unsigned char		buffer[PAGE_SIZE - sizeof(atomic_t) -
> +				       sizeof(struct irq_work)];
> +};
> +static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);


PAGE_SIZE isn't always 4K.

On typical powerpc systems this will give you 128K, and on some 512K, which is
probably not what we wanted.

The existing code just did:

#define NMI_BUF_SIZE           4096

So I think you should just go back to doing that.

cheers

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