Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > kref_put() could potentially add an unnecessary extra stack frame and would > > seem to be best avoided, though an optimising compiler ought to be able to > > inline if it can. > > If kref_put() is on your fast path, you have worse problems (kfree isn't > fast, right?) > > Anyway, it's an inline function, how can it add an extra stack frame? The call to the function pointer. Hopefully the compiler will optimise that away for an inlineable function. > > Are you now on the convert all refcounts to krefs path? > > "now"? Remember, I wrote kref all those years ago, Yes - and I thought it wasn't a good idea at the time. But this is the first time you've mentioned it to me, let alone pushed to change to it, that I recall. > everyone should use > it. It saves us having to audit the same pattern over and over again. > And, even nicer, it uses a refcount now, and as you are trying to > reference count an object, it is exactly what this was written for. > > So yes, I do think it should be used here, unless it is deemed to not > fit the pattern/usage model. kref_put() enforces a very specific destructor signature. I know of places where that doesn't work because the destructor takes more than one argument (granted that this is not the case here). So why does kref_put() exist at all? Why not kref_dec_and_test()? Why doesn't refcount_t get merged into kref, or vice versa? Having both would seem redundant. Mind you, I've been gradually reverting atomic_t-to-refcount_t conversions because it seems I'm not allowed refcount_inc/dec_return() and I want to get at the point refcount for tracing purposes. > > > > +module_exit(watch_queue_exit); > > > > > > module_misc_device()? > > > > warthog>git grep module_misc_device -- Documentation/ > > warthog1> > > Do I have to document all helper macros? If you add an API, documenting it is your privilege ;-) It's an important test of the API - if you can't describe it, it's probably wrong. Now I will grant that you didn't add that function... > Anyway, it saves you boilerplate code, but if built in, it's at the module > init level, not the fs init level, like you are asking for here. So that > might not work, it's your call. Actually, I probably shouldn't have a module exit function. It can't be a module as it's called by core code. I'll switch to builtin_misc_device(). > And how does the tracing and perf ring buffers do this without needing > volatile? Why not use the same type of interface they provide, as it's > always good to share code that has already had all of the nasty corner > cases worked out. I've no idea how trace does it - or even where - or even if. As far as I can see, grepping for mmap in kernel/trace/*, there's no mmap support. Reading Documentation/trace/ring-buffer-design.txt the trace subsystem has some sort of transient page fifo which is a lot more complicated than what I want and doesn't look like it'll be mmap'able. Looking at the perf ring buffer, there appears to be a missing barrier in perf_aux_output_end(): rb->user_page->aux_head = rb->aux_head; should be: smp_store_release(&rb->user_page->aux_head, rb->aux_head); It should also be using smp_load_acquire(). See Documentation/core-api/circular-buffers.rst And a (partial) patch has been proposed: https://lkml.org/lkml/2018/5/10/249 David
