On Fri, 15 Jul 2022 20:21:49 +0000
Song Liu <songliubraving@xxxxxx> wrote:
> >>> Wouldn't this need to be done anyway if BPF was first and live kernel
> >>> patching needed the update? An -EAGAIN would not suffice.
> >>
> >> prepare_direct_functions_for_ipmodify handles BPF-first-livepatch-later
> >> case. The benefit of prepare_direct_functions_for_ipmodify() is that it
> >> holds direct_mutex before ftrace_lock, and keeps holding it if necessary.
> >> This is enough to make sure we don't need the wash-rinse-repeat.
> >>
> >> OTOH, if we wait until __ftrace_hash_update_ipmodify(), we already hold
> >> ftrace_lock, but not direct_mutex. To make changes to bpf trampoline, we
> >> have to unlock ftrace_lock and lock direct_mutex to avoid deadlock.
> >> However, this means we will need the wash-rinse-repeat.
>
> What do you think about the prepare_direct_functions_for_ipmodify()
> approach? If this is not ideal, maybe we can simplify it so that it only
> holds direct_mutex (when necessary). The benefit is that we are sure
> direct_mutex is already held in __ftrace_hash_update_ipmodify(). However,
> I think it is not safe to unlock ftrace_lock in __ftrace_hash_update_ipmodify().
> We can get parallel do_for_each_ftrace_rec(), which is dangerous, no?
I'm fine with it. But one nit on the logic:
> int register_ftrace_function(struct ftrace_ops *ops)
> + __releases(&direct_mutex)
> {
> + bool direct_mutex_locked;
> int ret;
>
> ftrace_ops_init(ops);
>
> + ret = prepare_direct_functions_for_ipmodify(ops);
> + if (ret < 0)
> + return ret;
> +
> + direct_mutex_locked = ret == 1;
> +
Please make the above:
if (ret < 0)
return ret;
else if (ret == 1)
direct_mutex_locked = true;
It's much easier to read that way.
-- Steve
> mutex_lock(&ftrace_lock);
>
> ret = ftrace_startup(ops, 0);
>
> mutex_unlock(&ftrace_lock);
>
> + if (direct_mutex_locked)
> + mutex_unlock(&direct_mutex);
> return ret;
> }
> EXPORT_SYMBOL_GPL(register_ftrace_function);
> --
