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

Ingo Molnar
2009-04-02T19:18:05Z

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