linux-kernel - Re: [patch 04/18] x86, bts: wait until traced task has been scheduled out

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Message-ID: <20090402191730.GD843@elte.hu>
Date:	Thu, 2 Apr 2009 21:17:30 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	markus.t.metzger@...el.com
Cc:	tglx@...utronix.de, hpa@...or.com, markus.t.metzger@...il.com,
	roland@...hat.com, eranian@...glemail.com, oleg@...hat.com,
	juan.villacis@...el.com, ak@...ux.jf.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 04/18] x86, bts: wait until traced task has been
	scheduled out


* markus.t.metzger@...el.com <markus.t.metzger@...el.com> wrote:

> In order to stop branch tracing for a running task, we need to 
> first clear the branch tracing control bits before we may free the 
> tracing buffer. If the traced task is running, the cpu might still 
> trace that task after the branch trace control bits have cleared.
> 
> Wait until the traced task has been scheduled out before 
> proceeding.
> 
> A similar problem affects the task debug store context. We first 
> remove the context, then we need to wait until the task has been 
> scheduled out before we can free the context memory.
> 
> 
> Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Markus Metzger <markus.t.metzger@...el.com>
> ---
>  arch/x86/kernel/ds.c |   40 	40 +	0 -	0 !
>  1 file changed, 40 insertions(+)
> 
> Index: b/arch/x86/kernel/ds.c
> ===================================================================
> --- a/arch/x86/kernel/ds.c
> +++ b/arch/x86/kernel/ds.c
> @@ -250,6 +250,40 @@ static DEFINE_PER_CPU(struct ds_context 
>  #define system_context per_cpu(system_context_array, smp_processor_id())
>  
>  
> +/*
> + * Wait for the traced task to unschedule.
> + *
> + * This guarantees that the bts trace configuration has been
> + * synchronized with the cpu executing the task.
> + */
> +static void wait_to_unschedule(struct task_struct *task)
> +{

this should be in sched.c and task_is_running() should not be 
exported from there.

I.e. your original patch which i objected to is probably the right 
one, but should be named something like "task_wait_context_switch()" 
- which signals its purpose: that it is to wait for the task to 
context-switch at least once, so that its ptrace state is installed 
(or deinstalled) for sure.

	Ingo
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