Re: [PATCH] kprobes: Make optprobe optimizer multi-generational and asynchronous

[Andrea Parri]

Hi Masami,

> -	/* Step 5: Kick optimizer again if needed. But if there is a flush requested, */
> -	if (completion_done(&optimizer_completion))
> -		complete(&optimizer_completion);
> +	/* Step 2: Dispatch waiting room generation if allowed */
> +	optprobe_dispatch_generation();
>  
> -	if (!list_empty(&optimizing_list) || !list_empty(&unoptimizing_list))
> -		kick_kprobe_optimizer();	/*normal kick*/
> +	/* Step 3: Check completion if flush was requested */
> +	if (!optprobe_optimizer_busy()) {
> +		if (optprobe_flush_requested) {
> +			optprobe_flush_requested = false;
> +			complete_all(&optimizer_completion);
> +		}

Removing the completion_done() check also fixes a hang in the current
code: completion_done() is only true once the completion has been
completed, so complete() is never called while a flusher waits.  For
example, "echo 0 > /proc/sys/debug/kprobes-optimization" with an
optimized kprobe never returns.  I posted a fix for that separately [1].

[...]


> -	while (!list_empty(&optimizing_list) || !list_empty(&unoptimizing_list)) {
> +	while (optprobe_optimizer_busy()) {
>  		init_completion(&optimizer_completion);

This part is still racy, though.  The flusher drops kprobe_mutex while
it sleeps, and the sysctl handler and the debugfs "enabled" file do not
otherwise serialize against each other.  A second flusher can then run
init_completion() on the wait queue the first one is sleeping on: the
first waiter is dropped from the queue, and complete_all() wakes only
the second one.

With this patch the window is wider, since kprobe_mutex is released
while the Tasks RCU grace period runs and optprobe_optimizer_busy()
stays true until the generation is finalized.

The fix in [1] replaces the completion with a counter of optimizer
passes, bumped at the end of each pass and signalled with
wake_up_var_locked(), both under kprobe_mutex; the flusher samples the
count and waits with wait_var_event_mutex() until it changes.
Something similar should fit here, with the counter bumped at the end
of each kprobe_optimizer() call and the flusher re-checking
optprobe_optimizer_busy() under kprobe_mutex as it does now.

[1] https://lore.kernel.org/all/20260924092142.199198-1-parri.andrea@xxxxxxxxx/

Thanks,
  Andrea