[PATCH 7/9] ptrace: Simplify the wait_task_inactive call in ptrace_check_attach

[Peter Zijlstra]

On Wed, Apr 27, 2022 at 05:14:57PM +0200, Oleg Nesterov wrote:
> On 04/26, Eric W. Biederman wrote:
> >
> > Asking wait_task_inactive to verify that tsk->__state == __TASK_TRACED
> > was needed to detect the when ptrace_stop would decide not to stop
> > after calling "set_special_state(TASK_TRACED)".  With the recent
> > cleanups ptrace_stop will always stop after calling set_special_state.
> >
> > Take advatnage of this by no longer asking wait_task_inactive to
> > verify the state.  If a bug is hit and wait_task_inactive does not
> > succeed warn and return -ESRCH.
> 
> ACK, but I think that the changelog is wrong.
> 
> We could do this right after may_ptrace_stop() has gone. This doesn't
> depend on the previous changes in this series.

It very much does rely on there not being any blocking between
set_special_state() and schedule() tho. So all those PREEMPT_RT
spinlock->rt_mutex things need to be gone.

That is also the reason I couldn't do wait_task_inactive(task, 0) in the
other patch, I had to really match 'TASK_TRACED or TASK_FROZEN' any
other state must fail (specifically TASK_RTLOCK_WAIT must not match).