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SubjectRe: 5.8-rc*: kernel BUG at kernel/signal.c:1917
Date

On 20/07/20 12:26, peterz@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)));
> +

IIUC if the state changed and isn't TASK_RUNNING it *has* to have
__TASK_TRACED, so can't that be

SCHED_WARN_ON(tmp_state && !(tmp_state & __TASK_TRACED));

Other than that, LGTM.

> + 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 {

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