On Wed, 7 Jul 2021 22:42:47 +0800 Matt Wu <wuqiang.matt@xxxxxxxxxxxxx> wrote: > On 2021/7/7 PM9:29, Masami Hiramatsu wrote: > > On Wed, 7 Jul 2021 19:45:30 +0900 > > Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > >> On Wed, 7 Jul 2021 12:20:57 +0200 > >> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > >> > >>> On Wed, Jul 07, 2021 at 07:15:10PM +0900, Masami Hiramatsu wrote: > >>> > >>>> I actually don't want to keep this feature because no one use it. > >>>> (only systemtap needs it?) > >>> > >>> Yeah, you mentioned systemtap, but since that's out-of-tree I don't > >>> care. Their problem. > > > > Yeah, maybe it is not hard to update. > > > >>> > >>>> Anyway, if we keep the idea-level compatibility (not code level), > >>>> what we need is 'void *data' in the struct kretprobe_instance. > >>>> User who needs it can allocate their own instance data for their > >>>> kretprobes when initialising it and sets in their entry handler. > >>>> > >>>> Then we can have a simple kretprobe_instance. > >>> > >>> When would you do the alloc? When installing the retprobe, but that > >>> might be inside the allocator, which means you can't call the allocator > >>> etc.. :-) > >> > >> Yes, so the user may need to allocate a pool right before register_kretprobe(). > >> (whether per-kretprobe or per-task or global pool, that is user's choice.) > >> > >>> > >>> If we look at struct ftrace_ret_stack, it has a few fixed function > >>> fields. The calltime one is all that is needed for the kretprobe > >>> example code. > >> > >> kretprobe consumes 3 fields, a pointer to 'struct kretprobe' (which > >> stores callee function address in 'kretprobe::kp.addr'), a return > >> address and a frame pointer (*). > > > Oops, I forgot to add "void *data" for storing user data. > > > > Should use "struct kretprobe_holder *rph", since "struct kretprobe" belongs > to 3rd-party module (which might be unloaded any time). Good catch. Yes, instead of 'struct kretprobe', we need to use the holder. > User's own pool might not work if the module can be unloaded. Better manage > the pool in kretprobe_holder, which needs no changes from user side. No, since the 'data' will be only refered from user handler. If the kretprobe is released, then the kretprobe_holder will clear the refernce to the 'struct kretprobe'. Then, the user handler is never called. No one access the 'data'. Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
