On Wed, Sep 29, 2021 at 09:40:26PM +0200, Peter Zijlstra wrote: > On Wed, Sep 29, 2021 at 11:54:55AM -0700, Kees Cook wrote: > > > > > > > > It's supposed to show where a blocked task is blocked; the "wait > > > > > > > channel". > > > Since I think we're considering get_wchan() to be slow-path, can we just > > lock the runqueue and use arch_stack_walk_reliable()? > > Funny thing, when a task is blocked it isn't on the runqueue :-) > > So if all we want to do is capture a blocked task and fail otherwise we > don't need the rq->lock at all. > > Something like: > > unsigned long ip = 0; > > raw_spin_lock_irq(&p->pi_lock); > state = READ_ONCE(p->__state); > smp_rmb(); /* see try_to_wake_up() */ > if (state == TASK_RUNNING || state == TASK_WAKING || p->on_rq) > goto unlock; > > ip = /* do actual stack walk on a blocked task */ > unlock: > raw_spin_unlock_irq(&p->pi_lock); > > return ip; Ah, cool :-) I'd also add that I don't see any reason to use the "reliable" unwinding variant. AFAIK, just basic stack_trace_save_tsk() should be sufficient. -- Josh
