Messages in this thread |  | | Date | Sat, 19 Sep 2020 13:24:39 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] fgraph: Convert ret_stack tasklist scanning to rcu |
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On 09/18, Steven Rostedt wrote: > > On Mon, 7 Sep 2020 13:43:02 +0200 > Oleg Nesterov <oleg@redhat.com> wrote: > > > Afaics, with or without this change alloc_retstack_tasklist() can race > > with copy_process() and miss the new child; ftrace_graph_init_task() > > can't help, ftrace_graph_active can be set right after the check and > > for_each_process_thread() can't see the new process yet. > > There's a call in copy_process(): ftrace_graph_init_task() that initializes > a new tasks ret_stack,
Only if ftrace_graph_active != 0.
register_ftrace_graph() can increment ftrace_graph_active and call alloc_retstack_tasklist() right after ftrace_graph_init_task() checks ftrace_graph_active.
> and this loop will ignore it
and this loop won't see it unless the forking process finishes copy_process() and does list_add_tail_rcu(&p->tasks, &init_task.tasks) which makes it visible to for_each_process(). Yes, this is very unlikely.
> > Looks like, ftrace_graph_exit_task() was called by the exiting task > > in the past? Indeed, see 65afa5e603d50 ("tracing/function-return-tracer: > > free the return stack on free_task()"). I think it makes sense to > > simplify this function now, it can simply do kfree(t->ret_stack) and > > nothing more. > > Ah, yeah, then you are right. If it can't be called on a running task then > it can be simplified. Probably need a: > > WARN_ON_ONCE(t->on_rq); > > just to make sure this never happens.
Well, ftrace_graph_exit_task(t) is called by free_task(t), right before kmem_cache_free(t).
> > ACK, but ... > > > > > @@ -387,8 +387,8 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list) > > > } > > > } > > > > > > - read_lock(&tasklist_lock); > > > > then you should probably rename alloc_retstack_tasklist() ? > > > > tasklist, process thead? Is there a difference?
Aah, please ignore. Somehow I misinterpreted the _tasklist suffix, as if it refers to tasklist_lock.
Oleg.
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