[patch 4/14] x86, ds: wait before freeing the DS configuration

[patch 4/14] x86, ds: wait before freeing the DS configuration

[Markus Metzger]

After suspending bts tracing, the task is still traced as long as it is
running. The suspend will take effect when the task is scheduled out.

Wait until the traced task becomes inactive before freeing the DS configuration.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
---

Index: git-tip/arch/x86/kernel/ds.c
===================================================================
--- git-tip.orig/arch/x86/kernel/ds.c	2009-03-27 07:59:23.000000000 +0100
+++ git-tip/arch/x86/kernel/ds.c	2009-03-27 08:00:41.000000000 +0100
@@ -783,6 +783,14 @@ void ds_release_bts(struct bts_tracer *t
 
 	ds_suspend_bts(tracer);
 
+	/*
+	 * We must wait for the suspend to take effect before we may
+	 * free the tracer and the ds configuration.
+	 */
+	if (tracer->ds.context->task &&
+	    (tracer->ds.context->task != current))
+		wait_task_inactive(tracer->ds.context->task, 0);
+
 	WARN_ON_ONCE(tracer->ds.context->bts_master != tracer);
 	tracer->ds.context->bts_master = NULL;
 
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Re: [patch 4/14] x86, ds: wait before freeing the DS configuration

[Oleg Nesterov]

On 03/27, Markus Metzger wrote:
>
> --- git-tip.orig/arch/x86/kernel/ds.c	2009-03-27 07:59:23.000000000 +0100
> +++ git-tip/arch/x86/kernel/ds.c	2009-03-27 08:00:41.000000000 +0100
> @@ -783,6 +783,14 @@ void ds_release_bts(struct bts_tracer *t
>  
>  	ds_suspend_bts(tracer);
>  
> +	/*
> +	 * We must wait for the suspend to take effect before we may
> +	 * free the tracer and the ds configuration.
> +	 */
> +	if (tracer->ds.context->task &&
> +	    (tracer->ds.context->task != current))
> +		wait_task_inactive(tracer->ds.context->task, 0);
> +

Since you didn't replay yet, I will just repeat my question ;)

I am not sure I understand the problem. From the changelog:

        If the children are currently executing, the buffer
        may be freed while the hardware is still tracing.
        This might cause the hardware to overwrite memory.

So, the problem is that ds.context->task must not be running before we
can start to disable/free ds, yes? Something like ds_switch_to() should
be completed, right?

In that case I don't really understand how wait_task_inactive() can help.
If the task is killed it can be scheduled again, right after
wait_task_inactive() returns.

Also. This function is called from ptrace_bts_exit_tracer(), when the
tracee is not stopped. In this case wait_task_inactive() can spin forever.
For example, if the tracee simply does "for (;;) ;" it never succeeds.


If my understanding of the problem is wrong, could you please explain
it for dummies?

Oleg.

RE: [patch 4/14] x86, ds: wait before freeing the DS configuration

[Metzger, Markus T]

>-----Original Message-----
>From: Oleg Nesterov [mailto:oleg@redhat.com]
>Sent: Friday, March 27, 2009 3:36 PM
>To: Metzger, Markus T
>Cc: linux-kernel@vger.kernel.org; mingo@elte.hu; tglx@linutronix.de; hpa@zytor.com;
>markus.t.metzger@gmail.com; roland@redhat.com; eranian@googlemail.com; Villacis, Juan;
>ak@linux.jf.intel.com
>Subject: Re: [patch 4/14] x86, ds: wait before freeing the DS configuration
>
>On 03/27, Markus Metzger wrote:
>>
>> --- git-tip.orig/arch/x86/kernel/ds.c	2009-03-27 07:59:23.000000000 +0100
>> +++ git-tip/arch/x86/kernel/ds.c	2009-03-27 08:00:41.000000000 +0100
>> @@ -783,6 +783,14 @@ void ds_release_bts(struct bts_tracer *t
>>
>>  	ds_suspend_bts(tracer);
>>
>> +	/*
>> +	 * We must wait for the suspend to take effect before we may
>> +	 * free the tracer and the ds configuration.
>> +	 */
>> +	if (tracer->ds.context->task &&
>> +	    (tracer->ds.context->task != current))
>> +		wait_task_inactive(tracer->ds.context->task, 0);
>> +
>
>Since you didn't replay yet, I will just repeat my question ;)

I replied in http://lkml.org/lkml/2009/3/27/220.

regards,
markus.

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