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

    * 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


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