Messages in this thread | | | Date | Wed, 16 Mar 2022 09:23:19 +0100 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH] ptrace: fix ptrace vs tasklist_lock race on PREEMPT_RT. |
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On 2022-03-15 15:29:46 [+0100], Oleg Nesterov wrote: > > > > --- a/kernel/sched/core.c > > > > +++ b/kernel/sched/core.c > > > > @@ -3239,7 +3239,8 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state > > > > * is actually now running somewhere else! > > > > */ > > > > while (task_running(rq, p)) { > > > > - if (match_state && unlikely(READ_ONCE(p->__state) != match_state)) > > > > + if (match_state && > > > > + unlikely(!task_state_match_eq(p, match_state))) > > > > return 0; > > > > > > So wait_task_inactive() can return 0 but the task can run after that, right? > > > This is not what we want... > > > > Without checking both states you may never observe the requested state > > because it is set to TASK_RTLOCK_WAIT while waiting for a lock. Other > > than that, it may run briefly because it tries to acquire a lock or just > > acquired and this shouldn't be different from a task spinning on a lock. > > I don't understand. wait_task_inactive() is used to ensure that this task > doesn't and can't run again, until debugger resumes these tracee. > > Now. Unless I missed something, the tracee can leave CPU with saved_state > = TRACED (so task_state_match_eq() returns T) and wait_task_inactive() will > return. Then later the tracee will park in schedule again, yes. > > But, for example, what if debugger clears TIF_BLOCKSTEP in between, while > the tracee is running? Can't this race with __switch_to_xtra() ?
If you describe like that, then it appears better to only look at ->state. Otherwise, yes, you would see the expected state in ->saved_state and the task might still be on the CPU. Even if it is not actually running/ on the runqueue, it could be the case if the lock has been made available shortly after.
> Oleg.
Sebastian
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