Subject is missing patch number.
On Mon, 10 Feb 2014 16:38:56 +0100
Nicholas Mc Guire <der.herr@xxxxxxx> wrote:
>
> A lockfree approach to check_task_state
>
> This treates the state as an indicator variable and use it to probe
> saved_state lock free. There is actually no consistency demand on
> state/saved_state but rather a consistency demand on the transitions
> of the two variables but those transition, based on path inspection,
> are not independent.
>
> Its probably not faster than the lock/unlock case if uncontended - atleast
> it does not show up in benchmark results, but it would never be hit by a
> full pi-boost cycle as there is no contention.
>
> This also was tested against the test-case from Sebastian as well as
> rnning a few scripted gdb breakpoint debugging/single-stepping loops
> to trigger this.
To trigger what?
>
> Tested-by: Andreas Platschek <platschek@xxxxxxxxxxxxxxxx>
> Tested-by: Carsten Emde <C.Emde@xxxxxxxxx>
> Signed-off-by: Nicholas Mc Guire <der.herr@xxxxxxx>
> ---
> kernel/sched/core.c | 10 ++++++++--
> 1 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bf93f63..5690ba3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1074,11 +1074,17 @@ static int migration_cpu_stop(void *data);
> static bool check_task_state(struct task_struct *p, long match_state)
> {
> bool match = false;
> + long state, saved_state;
> +
> + /* catch restored state */
> + do {
> + state = p->state;
> + saved_state = p->saved_state;
> + rmb(); /* make sure we actually catch updates */
The problem I have with this is that there's no matching wmb(). Also,
shouldn't that be a smp_rmb(), I don't think we can race with devices
here.
> + } while (state != p->state);
>
> - raw_spin_lock_irq(&p->pi_lock);
> if (p->state == match_state || p->saved_state == match_state)
> match = true;
> - raw_spin_unlock_irq(&p->pi_lock);
>
> return match;
> }
In rtmutex.c we have:
pi_lock(&self->pi_lock);
__set_current_state(self->saved_state);
self->saved_state = TASK_RUNNING;
pi_unlock(&self->pi_lock);
As there is no wmb() here, it can be very possible that another CPU
will see saved_state as TASK_RUNNING, and current state as
TASK_RUNNING, and miss the update completely.
I would not want to add a wmb() unless there is a real bug with the
check state, as the above is in a very fast path and the check state is
in a slower path.
-- Steve
--
To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
