linux-kernel - Re: Q: user_enable_single_step() && update

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Message-ID: <20120801151442.GA9697@redhat.com>
Date:	Wed, 1 Aug 2012 17:14:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Roland McGrath <roland@...k.frob.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	ananth@...ibm.com, a.p.zijlstra@...llo.nl, mingo@...hat.com,
	srikar@...ux.vnet.ibm.com
Subject: Re: Q: user_enable_single_step() && update_debugctlmsr()

On 08/01, Sebastian Andrzej Siewior wrote:
>
> On 08/01/2012 05:01 PM, Oleg Nesterov wrote:
>> On 08/01, Sebastian Andrzej Siewior wrote:
>>> So a patch like
>>> --- a/arch/x86/kernel/step.c
>>> +++ b/arch/x86/kernel/step.c
>>> @@ -173,8 +173,8 @@ static void enable_step(struct task_struct *child,
>>> bool block)
>>>                  unsigned long debugctl = get_debugctlmsr();
>>>
>>>                  debugctl |= DEBUGCTLMSR_BTF;
>>> -               update_debugctlmsr(debugctl);
>>>                  set_tsk_thread_flag(child, TIF_BLOCKSTEP);
>>> +               update_debugctlmsr(debugctl);
>>>          } else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) {
>>>                  unsigned long debugctl = get_debugctlmsr();
>>>
>>> should fix the race
>>
>> No, I don't think it can fix something ;) or make any difference.
>
> Why? You _first_ set the task flag

Yes, and this task is "child".

> followed by the CPU register. Now
> switch_to() would see the bit set and act.

child sleeps and doesn't participate in switch_to(). Debugger and another
(unrelated) task do.

Oleg.

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