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

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Date:	Tue, 24 Feb 2015 08:27:19 +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}


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * 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?

[...]

> If we fix the deadlock potential, and get a true global 
> ordering of various oopses/warnings as they triggered (or 
> at least timestamping them), [...]

If we had a global 'trouble counter' we could use that to 
refine the spin-looping timeout: instead of using a pure 
timeout of a few seconds, we could say 'a timeout of a few 
seconds while the counter does not increase'.

I.e. only override the locking/ordering if the owner CPU 
does not seem to be able to make progress with printing the 
oops/warning.

Thanks,

	Ingo
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