----- On Nov 16, 2020, at 4:02 PM, rostedt rostedt@xxxxxxxxxxx wrote:
> On Mon, 16 Nov 2020 15:44:37 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
>> If you use a stub function, it shouldn't affect anything. And the worse
>> that would happen is that you have a slight overhead of calling the stub
>> until you can properly remove the callback.
>
> Something like this:
>
> (haven't compiled it yet, I'm about to though).
>
> -- Steve
>
> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> index 3f659f855074..8eab40f9d388 100644
> --- a/kernel/tracepoint.c
> +++ b/kernel/tracepoint.c
> @@ -53,10 +53,16 @@ struct tp_probes {
> struct tracepoint_func probes[];
> };
>
> -static inline void *allocate_probes(int count)
> +/* Called in removal of a func but failed to allocate a new tp_funcs */
> +static void tp_stub_func(void)
I'm still not sure whether it's OK to call a (void) function with arguments.
> +{
> + return;
> +}
> +
> +static inline void *allocate_probes(int count, gfp_t extra_flags)
> {
> struct tp_probes *p = kmalloc(struct_size(p, probes, count),
> - GFP_KERNEL);
> + GFP_KERNEL | extra_flags);
> return p == NULL ? NULL : p->probes;
> }
>
> @@ -150,7 +156,7 @@ func_add(struct tracepoint_func **funcs, struct
> tracepoint_func *tp_func,
> }
> }
> /* + 2 : one for new probe, one for NULL func */
> - new = allocate_probes(nr_probes + 2);
> + new = allocate_probes(nr_probes + 2, 0);
> if (new == NULL)
> return ERR_PTR(-ENOMEM);
> if (old) {
> @@ -188,8 +194,9 @@ static void *func_remove(struct tracepoint_func **funcs,
> /* (N -> M), (N > 1, M >= 0) probes */
> if (tp_func->func) {
> for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
> - if (old[nr_probes].func == tp_func->func &&
> - old[nr_probes].data == tp_func->data)
> + if ((old[nr_probes].func == tp_func->func &&
> + old[nr_probes].data == tp_func->data) ||
> + old[nr_probes].func == tp_stub_func)
> nr_del++;
> }
> }
> @@ -207,15 +214,20 @@ static void *func_remove(struct tracepoint_func **funcs,
> int j = 0;
> /* N -> M, (N > 1, M > 0) */
> /* + 1 for NULL */
> - new = allocate_probes(nr_probes - nr_del + 1);
> - if (new == NULL)
> - return ERR_PTR(-ENOMEM);
> - for (i = 0; old[i].func; i++)
> - if (old[i].func != tp_func->func
> - || old[i].data != tp_func->data)
> - new[j++] = old[i];
> - new[nr_probes - nr_del].func = NULL;
> - *funcs = new;
> + new = allocate_probes(nr_probes - nr_del + 1, __GFP_NOFAIL);
> + if (new) {
> + for (i = 0; old[i].func; i++)
> + if (old[i].func != tp_func->func
> + || old[i].data != tp_func->data)
as you point out in your reply, skip tp_stub_func here too.
> + new[j++] = old[i];
> + new[nr_probes - nr_del].func = NULL;
> + } else {
> + for (i = 0; old[i].func; i++)
> + if (old[i].func == tp_func->func &&
> + old[i].data == tp_func->data)
> + old[i].func = tp_stub_func;
I think you'll want a WRITE_ONCE(old[i].func, tp_stub_func) here, matched
with a READ_ONCE() in __DO_TRACE. This introduces a new situation where the
func pointer can be updated and loaded concurrently.
> + }
> + *funcs = old;
The line above seems wrong for the successful allocate_probe case. You will likely
want *funcs = new on successful allocation, and *funcs = old for the failure case.
Thanks,
Mathieu
> }
> debug_print_probes(*funcs);
> return old;
> @@ -300,6 +312,10 @@ static int tracepoint_remove_func(struct tracepoint *tp,
> return PTR_ERR(old);
> }
>
> + if (tp_funcs == old)
> + /* Failed allocating new tp_funcs, replaced func with stub */
> + return 0;
> +
> if (!tp_funcs) {
> /* Removed last function */
> if (tp->unregfunc && static_key_enabled(&tp->key))
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
