linux-kernel - Re: [PATCH 1/7] Add die_spin_lock

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Message-ID: <20150224064107.GB15387@gmail.com>
Date:	Tue, 24 Feb 2015 07:41:07 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	sam.bobroff@....ibm.com, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
	Russell King <linux@....linux.org.uk>, peterz@...radead.org,
	Don Zickus <dzickus@...hat.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/7] Add die_spin_lock_{irqsave,irqrestore}


* Anton Blanchard <anton@...ba.org> wrote:

> +static arch_spinlock_t die_lock = __ARCH_SPIN_LOCK_UNLOCKED;
> +static int die_owner = -1;
> +static unsigned int die_nest_count;
> +
> +unsigned long __die_spin_lock_irqsave(void)
> +{
> +	unsigned long flags;
> +	int cpu;
> +
> +	/* racy, but better than risking deadlock. */
> +	raw_local_irq_save(flags);
> +
> +	cpu = smp_processor_id();
> +	if (!arch_spin_trylock(&die_lock)) {
> +		if (cpu != die_owner)
> +			arch_spin_lock(&die_lock);

So why not trylock and time out here after a few seconds, 
instead of indefinitely supressing some potentially vital 
output due to some other CPU crashing/locking with the lock 
held?

> +	}
> +	die_nest_count++;
> +	die_owner = cpu;
> +
> +	return flags;

I suspect this would work in most cases.

If we fix the deadlock potential, and get a true global 
ordering of various oopses/warnings as they triggered (or 
at least timestamping them), then I'm sold on this I guess, 
it will likely improve things.

Thanks,

	Ingo
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