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From: "Shi, Yang" <yang.shi@linaro.org>
To: Will Deacon <will.deacon@arm.com>, James Morse <james.morse@arm.com>
Cc: catalin.marinas@arm.com, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linaro-kernel@lists.linaro.org
Subject: Re: [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump
Date: Wed, 10 Feb 2016 10:12:11 -0800	[thread overview]
Message-ID: <56BB7D7B.4060002@linaro.org> (raw)
In-Reply-To: <20160210121030.GH1052@arm.com>

On 2/10/2016 4:10 AM, Will Deacon wrote:
> 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());

One follow up question, is it possible to have both tsk != current and 
on_irq_stack is true at the same time? If it is possible, this may be a 
problem in unwind_frame called by profile_pc which has tsk being NULL.

Thanks,
Yang

>> 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
>

WARNING: multiple messages have this Message-ID (diff)
From: yang.shi@linaro.org (Shi, Yang)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump
Date: Wed, 10 Feb 2016 10:12:11 -0800	[thread overview]
Message-ID: <56BB7D7B.4060002@linaro.org> (raw)
In-Reply-To: <20160210121030.GH1052@arm.com>

On 2/10/2016 4:10 AM, Will Deacon wrote:
> 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());

One follow up question, is it possible to have both tsk != current and 
on_irq_stack is true at the same time? If it is possible, this may be a 
problem in unwind_frame called by profile_pc which has tsk being NULL.

Thanks,
Yang

>> 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
>

  reply	other threads:[~2016-02-10 18:12 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-02-09 21:26 [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump Yang Shi
2016-02-09 21:26 ` Yang Shi
2016-02-10 10:29 ` Will Deacon
2016-02-10 10:29   ` Will Deacon
2016-02-10 11:52   ` James Morse
2016-02-10 11:52     ` James Morse
2016-02-10 12:10     ` Will Deacon
2016-02-10 12:10       ` Will Deacon
2016-02-10 18:12       ` Shi, Yang [this message]
2016-02-10 18:12         ` Shi, Yang
2016-02-11 10:41         ` James Morse
2016-02-11 10:41           ` James Morse
2016-02-11 17:36           ` Shi, Yang
2016-02-11 17:36             ` Shi, Yang

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