linux-kernel - Re: [PATCH] arm64: reenable interrupt when handling ptrace breakpoint

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Date:	Wed, 13 Jan 2016 09:17:46 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Catalin.Marinas@....com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH] arm64: reenable interrupt when handling ptrace breakpoint

On 1/13/2016 2:26 AM, Will Deacon wrote:
> On Tue, Jan 12, 2016 at 11:59:54AM -0800, Shi, Yang wrote:
>> On 12/21/2015 9:00 AM, Will Deacon wrote:
>>> On Mon, Dec 21, 2015 at 05:51:22PM +0100, Thomas Gleixner wrote:
>>>> On Mon, 21 Dec 2015, Will Deacon wrote:
>>>>> +static void send_user_sigtrap(int si_code)
>>>>> +{
>>>>> +	struct pt_regs *regs = current_pt_regs();
>>>>> +	siginfo_t info = {
>>>>> +		.si_signo	= SIGTRAP,
>>>>> +		.si_errno	= 0,
>>>>> +		.si_code	= si_code,
>>>>> +		.si_addr	= (void __user *)instruction_pointer(regs),
>>>>> +	};
>>>>> +
>>>>> +	if (WARN_ON(!user_mode(regs)))
>>>>> +		return;
>>>>> +
>>>>> +	preempt_disable();
>>>>
>>>> That doesn't work on RT either. force_sig_info() takes task->sighand->siglock,
>>>> which is a 'sleeping' spinlock on RT.
>>>
>>> Ah, I missed that :/
>>>
>>>> Why would we need to disable preemption here at all? What's the problem of
>>>> being preempted or even migrated?
>>>
>>> There *might* not be a problem, I'm just really nervous about changing
>>> the behaviour on the debug path and subtly changing how ptrace behaves.
>>>
>>> My worry was that you could somehow get back into the tracer, and it
>>> could remove a software breakpoint in the knowledge that it wouldn't
>>> see any future (spurious) SIGTRAPs for that location.
>>>
>>> Without a concrete example, however, I guess I'll bite the bullet and
>>> enable irqs across the call to force_sig_info, since there is clearly a
>>> real issue here on RT.
>>
>> This might be buried in email storm during the holiday. Just want to double
>> check the status. I'm supposed there is no objection for getting it merged
>> in upstream?
>
> Sorry, when you replied with:
>
>> I think we could just extend the "signal delay send" approach from x86-64
>> to arm64, which is currently used by x86-64 on -rt kernel only.
>
> I understood that you were going to fix -rt, so I dropped this pending
> anything more from you.
>
> What's the plan?

Sorry for the confusion. The "signal delay send" approach used by x86-64 
-rt should be not necessary for arm64 right now. Reenabling interrupt is 
still the preferred approach.

Since x86-64 has per-CPU IST exception stack, so preemption has to be 
disabled all the time. However, it is not applicable to other 
architectures for now, including arm64.

Thanks,
Yang

>
> Will
>