On 20/07/20 14:17, pet...@infradead.org wrote:
> On Mon, Jul 20, 2020 at 01:20:26PM +0100, Valentin Schneider wrote:
>> On 20/07/20 12:26, pet...@infradead.org wrote:
>
>> > + /*
>> > + * 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));
>
> Suppose task->state == TASK_UNINTERRUPTIBLE, and task != current, and
> then someone goes and does task->state = __TASK_TRACED.
>
> That is, your statement is correct given the current code, but we also
> want to verify no new code comes along and does something 'creative'.
>
That is what I was trying to go for; AFAICT your approach only warns if
__TASK_TRACED gets removed between the two loads (IOW it was already
there). The way I was seeing it is:
- We only get here if prev_state != tmp_state; IOW we know we raced with
something
- if (tmp_state), then it wasn't with ttwu_remote()
- thus it must only be with ptrace shenanigans, IOW __TASK_TRACED must be
there.
Now, what I suggested still doesn't detect what you pointed out, or some
crazier thing that sets __TASK_TRACED *and* some other stuff. IIUC the
ptrace transformation does TASK_TRACED -> __TASK_TRACED, so we could have
it as:
/* TODO: name me */
#define foobar TASK_TRACED ^ __TASK_TRACED
...
/* not TASK_RUNNING; check against allowed transformations
SCHED_WARN_ON(tmp_state && ((prev_state ^ tmp_state) & ~foobar));
That said...
> Or is the heat getting to me?
... that may apply to me as well :-)
