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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20190529230954.GA3164@kroah.com> References: <20190529230954.GA3164@kroah.com> <20190528231218.GA28384@kroah.com> <20190528162603.GA24097@kroah.com> <155905930702.7587.7100265859075976147.stgit@warthog.procyon.org.uk> <155905931502.7587.11705449537368497489.stgit@warthog.procyon.org.uk> <4031.1559064620@warthog.procyon.org.uk> <31936.1559146000@warthog.procyon.org.uk> To: Greg KH Cc: dhowells@redhat.com, viro@zeniv.linux.org.uk, raven@themaw.net, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-block@vger.kernel.org, keyrings@vger.kernel.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <24719.1559306541.1@warthog.procyon.org.uk> Content-Transfer-Encoding: 8BIT Date: Fri, 31 May 2019 13:42:21 +0100 Message-ID: <24720.1559306541@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Fri, 31 May 2019 12:42:32 +0000 (UTC) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Greg KH 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