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Date: Wed, 1 Aug 2012 17:14:42 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Roland McGrath <roland@hack.frob.com>, "H. Peter Anvin" <hpa@zytor.com>,
        linux-kernel@vger.kernel.org, ananth@in.ibm.com,
        a.p.zijlstra@chello.nl, mingo@redhat.com, srikar@linux.vnet.ibm.com
Subject: Re: Q: user_enable_single_step() && update_debugctlmsr()
Message-ID: <20120801151442.GA9697@redhat.com>
References: <20120801130118.GA2386@redhat.com> <50192FF5.1060208@linutronix.de> <20120801134652.GA4707@redhat.com> <50193528.3070503@linutronix.de> <20120801140121.GA5333@redhat.com> <50193B5C.90404@linutronix.de> <20120801143132.GA7550@redhat.com> <5019427B.1070002@linutronix.de> <20120801150119.GA8845@redhat.com> <50194750.2050601@linutronix.de>
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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.