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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Tue, 24 Feb 2015 08:27:19 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Anton Blanchard <anton@samba.org>
Subject: Re: [PATCH 1/7] Add die_spin_lock_{irqsave,irqrestore}
Message-ID: <20150224072719.GB15894@gmail.com>
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Cc: Don Zickus <dzickus@redhat.com>, x86@kernel.org,
 Russell King <linux@arm.linux.org.uk>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
 peterz@infradead.org, hpa@zytor.com, linux-kernel@vger.kernel.org,
 Steven Rostedt <rostedt@goodmis.org>,
 Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@redhat.com>,
 Paul Mackerras <paulus@samba.org>, linux-arm-kernel@lists.infradead.org,
 Andrew Morton <akpm@linux-foundation.org>, linuxppc-dev@lists.ozlabs.org,
 Thomas Gleixner <tglx@linutronix.de>, sam.bobroff@au1.ibm.com,
 Arjan van de Ven <arjan@infradead.org>
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* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Anton Blanchard <anton@samba.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