linux-kernel - Re: [PATCH] arm64: use raw_smp_processor

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Date:	Wed, 10 Feb 2016 12:10:30 +0000
From:	Will Deacon <will.deacon@....com>
To:	James Morse <james.morse@....com>
Cc:	Yang Shi <yang.shi@...aro.org>, catalin.marinas@....com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump

On Wed, Feb 10, 2016 at 11:52:31AM +0000, James Morse wrote:
> On 10/02/16 10:29, Will Deacon wrote:
> > On Tue, Feb 09, 2016 at 01:26:22PM -0800, Yang Shi wrote:
> >> dump_backtrace may be called in kthread context, which is not bound to a single
> >> cpu, i.e. khungtaskd, then calling smp_processor_id may trigger the below bug
> >> report:
> > 
> > If we're preemptible here, it means that our irq_stack_ptr is potentially
> > bogus. Whilst this isn't an issue for kthreads, it does feel like we
> > could make this slightly more robust in the face of potential frame
> > corruption. Maybe just zero the IRQ stack pointer if we're in preemptible
> > context?
> 
> Switching between stacks is only valid if we are tracing ourselves while on the
> irq_stack, we should probably prevent it for other tasks too.
> 
> Something like (untested):
> ---------------------
> if (tsk == current && in_atomic())
> 	irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
> else
> 	irq_stack_ptr = 0;
> ---------------------
> 
> This would work when we trace ourselves while on the irq_stack, but break*
> tracing a running task on a remote cpu (khungtaskd doesn't do this).
> 
> The same fix would apply to unwind_frame(), we have 'tsk' in both functions.
> 
> Thoughts?

in_atomic is a misnomer:

  https://lwn.net/Articles/274695/

;)

So we might be better off zeroing the pointer if tsk != current ||
preemptible(). But yeah, I think we're in general agreement about this.

Will