On Mon, Jul 20, 2020 at 12:59:24PM +0200, pet...@infradead.org wrote:
> On Mon, Jul 20, 2020 at 10:41:06AM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 20, 2020 at 10:26:58AM +0200, Oleg Nesterov wrote:
> > > Peter,
> > >
> > > Let me add another note. TASK_TRACED/TASK_STOPPED was always protected by
> > > ->siglock. In particular, ttwu(__TASK_TRACED) must be always called with
> > > ->siglock held. That is why ptrace_freeze_traced() assumes it can safely
> > > do s/TASK_TRACED/__TASK_TRACED/ under spin_lock(siglock).
> > >
> > > Can this change race with
> > >
> > > if (signal_pending_state(prev->state, prev)) {
> > > prev->state = TASK_RUNNING;
> > > }
> > >
> > > in __schedule() ? Hopefully not, signal-state is protected by siglock too.
> > >
> > > So I think this logic was correct even if it doesn't look nice. But
> > > "doesn't
> > > look nice" is true for the whole ptrace code ;)
> >
> > *groan*... another bit of obscure magic :-(
> >
> > let me go try and wake up and figure out how best to deal with this.
This then? That seems to survive the strace thing.
---
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;
+ }
+
/* 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 {
