linux-kernel - Re: BUG: using smp_processor_id() during suspend with 2.6.25-rc8

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Date:	Tue, 8 Apr 2008 00:56:32 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Zdenek Kabelac <zdenek.kabelac@...il.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: BUG: using smp_processor_id() during suspend with 2.6.25-rc8

On Tuesday, 8 of April 2008, Jiri Kosina wrote:
> On Tue, 8 Apr 2008, Rafael J. Wysocki wrote:
> 
> > > The mce resume is a sysdev.
> > > sysdevs were always supposed to run completely with interrupts off. If they 
> > > don't anymore that's some kind of higher level resume code bug which you need 
> > > to fix there, not hack around in the low level code.
> > They are executed with interrupts disabled, on one CPU.
> 
> So, any idea why mce_resume() -> mce_init() -> debug_smp_processor_id() 
> triggers the warning? Apparently preempt_count is zero, irqs_disabled() 
> returns false, and cpumask_of_cpu() is not equal to current->cpus_allowed.
> 
> So there clearly is a bug somewhere.

Yes, there is.  Still, I wonder why doesn't everyone see it.
 
> > > Obviously turning on preemption anywhere around the machine check is
> > > fatal because it touches CPU state and if you reschedule you could
> > > switch to another CPU and change or access the wrong CPU's state.
> > FWIW, at the point when sysdevs are resumed we are single-threaded.
> 
> Is that really relevant here? We still could be switched over to another 
> CPU, and that would break things.

No, we couldn't, because the other CPUs are off at this point.

Thanks,
Rafael
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