On Mon, Jul 20, 2020 at 01:26:23PM +0200, pet...@infradead.org wrote:
> kernel/sched/core.c | 34 ++++++++++++++++++++++++++++------
> 1 file changed, 28 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e15543cb84812..b5973d7fa521c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4100,9 +4100,9 @@ pick_next_task(struct rq *rq, struct task_struct *prev,
> struct rq_flags *rf)
> */
> static void __sched notrace __schedule(bool preempt)
> {
> + unsigned long prev_state, tmp_state;
> struct task_struct *prev, *next;
> unsigned long *switch_count;
> - unsigned long prev_state;
> struct rq_flags rf;
> struct rq *rq;
> int cpu;
> @@ -4140,16 +4140,38 @@ static void __sched notrace __schedule(bool preempt)
> rq_lock(rq, &rf);
> smp_mb__after_spinlock();
>
> + /*
> + * We must re-load prev->state in case ttwu_remote() changed it
> + * before we acquired rq->lock.
> + */
> + tmp_state = prev->state;
> + if (unlikely(prev_state != tmp_state)) {
> + /*
> + * ptrace_{,un}freeze_traced() think it is cool to change
> + * ->state around behind our backs between TASK_TRACED and
> + * __TASK_TRACED.
> + *
> + * This is safe because this, as well as any __TASK_TRACED
> + * wakeups are under siglock.
> + *
> + * For any other case, a changed prev_state must be to
> + * TASK_RUNNING, such that when it blocks, the load has
> + * happened before the smp_mb().
> + *
> + * Also see the comment with deactivate_task().
> + */
> + SCHED_WARN_ON(tmp_state && (prev_state & __TASK_TRACED &&
> + !(tmp_state & __TASK_TRACED)));
> +
> + prev_state = tmp_state;
While trying to write a changelog for this thing, I can't convince
myself we don't need:
smp_mb();
here. Consider:
CPU0 CPU1 CPU2
schedule()
prev_state = prev->state;
spin_lock(rq->lock);
smp_mb__after_spin_lock();
ptrace_freeze_traced()
spin_lock(siglock)
task->state = __TASK_TRACED;
spin_unlock(siglock);
tmp_state = prev->state;
if (prev_state != tmp_state)
prev_state = tmp_state;
/* NO SMP_MB */
if (prev_state)
deactivate_task()
prev->on_rq = 0;
spin_lock(siglock);
ttwu()
if (rq->on_rq
&& ...)
goto unlock;
smp_acquire__after_ctrl_dep();
p->state =
TASK_WAKING;
Looses the ordering we previously relied upon. That is, CPU1's
prev->state load and prev->on_rq store can get reordered vs CPU2.
OTOH, we have a control dependency on CPU1 as well, that should provide
LOAD->STORE ordering, after all we only do the ->on_rq=0 store, IFF we
see prev_state.
So that is:
if (p->state) if (!p->on_rq)
p->on_rq = 0; p->state = TASK_WAKING
which matches a CTRL-DEP to a CTRL-DEP ...
But this then means we can simplify dbfb089d360 as well, but now my head
hurts.
> + }
> +
> /* Promote REQ to ACT */
> rq->clock_update_flags <<= 1;
> update_rq_clock(rq);
>
> switch_count = &prev->nivcsw;
> - /*
> - * We must re-load prev->state in case ttwu_remote() changed it
> - * before we acquired rq->lock.
> - */
> - if (!preempt && prev_state && prev_state == prev->state) {
> + if (!preempt && prev_state) {
> if (signal_pending_state(prev_state, prev)) {
> prev->state = TASK_RUNNING;
> } else {
