linux-kernel - Re: [PATCH] fgraph: Convert ret_stack tasklist scanning to rcu

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Date:   Mon, 7 Sep 2020 13:43:02 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     rostedt@...dmis.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] fgraph: Convert ret_stack tasklist scanning to rcu

On 09/06, Davidlohr Bueso wrote:
>
> Here tasklist_lock does not protect anything other than the list
> against concurrent fork/exit. And considering that the whole thing
> is capped by FTRACE_RETSTACK_ALLOC_SIZE (32), it should not be a
> problem to have a pontentially stale, yet stable, list. The task cannot
> go away either, so we don't risk racing with ftrace_graph_exit_task()
> which clears the retstack.

I don't understand this code but I think you right, tasklist_lock buys
nothing.

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.

This can't race with ftrace_graph_exit_task(), it is called after the
full gp pass. But this function looks very confusing to me, I don't
understand the barrier and the "NULL must become visible to IRQs before
we free it" comment.

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.

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() ?

Oleg.