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From:   David Howells <dhowells@redhat.com>
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To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     dhowells@redhat.com, viro@zeniv.linux.org.uk, raven@themaw.net,
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Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer
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Date:   Fri, 31 May 2019 13:42:21 +0100
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Greg KH <gregkh@linuxfoundation.org> wrote:

> > 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()?
>
> The destructor only takes one object pointer as you are finally freeing
> that object.  What more do you need/want to "know" at that point in
> time?

Imagine that I have an object that's on a list rooted in a namespace and that
I have a lot of these objects.  Imagine further that any time I want to put a
ref on one of these objects, it's in a context that has the namespace pinned.
I therefore don't need to store a pointer to the namespace in every object
because I can pass that in to the put function

Indeed, I can still access the namespace even after the decrement didn't
reduce the usage count to 0 - say for doing statistics.

> What would kref_dec_and_test() be needed for?

Why do you need kref_put() to take a destructor function pointer?  Why cannot
that be replaced with, say:

	static inline bool __kref_put(struct kref *k)
	{
		return refcount_dec_and_test(&k->refcount);
	}

and then one could do:

	void put_foo(struct foo_net *ns, struct foo *f)
	{
		if (__kref_put(&f->refcount)) {
			// destroy foo
		}
	}

that way the destruction code does not have to be offloaded into its own
function and you still have your pattern to look for.

For tracing purposes, I could live with something like:

	static inline
	bool __kref_put_return(struct kref *k, unsigned int *_usage)
	{
		return refcount_dec_and_test_return(&k->refcount, _usage);
	}

and then I could do:

	void put_foo(struct foo_net *ns, struct foo *f)
	{
		unsigned int u;
		bool is_zero = __kref_put_return(&f->refcount, &u);

		trace_foo_refcount(f, u);
		if (is_zero) {
			// destroy foo
		}
	}

then it could be made such that you can disable the ability of
refcount_dec_and_test_return() to pass back a useful refcount value if you
want a bit of extra speed.

Or even if refcount_dec_return() is guaranteed to return 0 if the count hits
the floor and non-zero otherwise and there's a config switch to impose a
stronger guarantee that it will return a value that's appropriately
transformed to look as if I was using atomic_dec_return().

Similarly for refcount_inc_return() - it could just return gibberish unless
the same config switch is enabled.

Question for AMD/Intel guys: I'm curious if LOCK DECL faster than LOCK XADD -1
on x86_64?

> > Why doesn't refcount_t get merged into kref, or vice versa?  Having both
> > would seem redundant.
>
> kref uses refcount_t and provides a different functionality on top of
> it.  Not all uses of a refcount in the kernel is for object lifecycle
> reference counting, as you know :)

I do?  I can't think of one offhand.  Not that I'm saying you're wrong on
that - there's an awful lot of kernel.

David