Date: Sat, 25 Oct 2014 15:47:02 -0700 From: John-Mark Gurney <jmg@funkthat.com> To: Mateusz Guzik <mjguzik@gmail.com> Cc: freebsd-arch@freebsd.org Subject: Re: refcount_release_take_##lock Message-ID: <20141025224702.GY82214@funkthat.com> In-Reply-To: <20141025201240.GC19066@dft-labs.eu> References: <20141025184448.GA19066@dft-labs.eu> <20141025190407.GU82214@funkthat.com> <20141025192632.GB19066@dft-labs.eu> <20141025195334.GW82214@funkthat.com> <20141025201240.GC19066@dft-labs.eu>
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Mateusz Guzik wrote this message on Sat, Oct 25, 2014 at 22:12 +0200: > On Sat, Oct 25, 2014 at 12:53:34PM -0700, John-Mark Gurney wrote: > > Mateusz Guzik wrote this message on Sat, Oct 25, 2014 at 21:26 +0200: > > > On Sat, Oct 25, 2014 at 12:04:07PM -0700, John-Mark Gurney wrote: > > > > Mateusz Guzik wrote this message on Sat, Oct 25, 2014 at 20:44 +0200: > > > > > The following idiom is used here and there: > > > > > > > > > > int old; > > > > > old = obj->ref; > > > > > if (old > 1 && atomic_cmpset_int(&obj->ref, old, old -1)) > > > > > return; > > > > > lock(&something); > > > > > if (refcount_release(&obj->ref) == 0) { > > > > > unlock(&something); > > > > > return; > > > > > } > > > > > free up > > > > > unlock(&something); > > > > > > > > > > ========== > > > > > > > > Couldn't this be better written as: > > > > if (__predict_false(refcount_release(&obj->ref) == 0)) { > > > > lock(&something); > > > > if (__predict_true(!obj->ref)) { > > > > free up > > > > } > > > > unlock(&something); > > > > } > > > > > > > > The reason I'm asking is that I changed how IPsec SA ref counting was > > > > handled, and used something similar... > > > > > > > > My code gets rid of a branch, and is better in that it uses refcount > > > > API properly, instead of using atomic_cmpset_int... > > > > > > This is used when given obj is kept on a list and code which traverses > > > it (locked) expects found objects to be valid to ref. > > > > > > If we were to reach count of 0 and then lock, it would be possible that > > > other thread refed + unrefed the object and is now trying to lock as > > > well. > > > > Per the email I wrote to Ian, this "assumption" needs to be well > > documented that though the "list" has a reference, and that this > > reference is not accounted for in the ref count... > > > > And I personally think that it's a bug for the list to not hold it's > > own reference... Yes, then you need to compare for when the ref count > > hits one, and do the lock/dec/free/unlock, but that keeps the refcount > > sane... > > Well, this is for stuff which cleans up after itself. I hope that everything cleans up after itself.. :) otherwise we'd have memory leaks everywhere... > Example usage is with per-uid stats for resource limits. These > automatically free themselves with the last cred with given uid. This example doesn't give me enough information to decide what you mean.. Is there a hash table that we look up these cred structures? or are they referenced from an already existing reference? > This has its own problems (like constant creation and destruction of > stuff for the same cred), but seems ok enough for some cases. > > Otherwise we would have to actively free these structs somehow. I'm still not sure how your example addresses this.. I believe you wrote the data structure case, but it wasn't clear that is what you were doing, and as I said, I think it's a bug to have an implicit ref in such data structures w/o properly documenting them... Part of the reason why we need documentation to make sure people don't make mistakes like these... > > > That could be remedied for type stable object by having a generation > > > counter, but I doubt it's worth it. Not to mention objects we lock here > > > are freeable :) > > > > That's too heavy weight... > > > > > > > I decided to implement it as a common function. > > > > > > > > > > We have only refcount.h and I didn't want to bloat all including code > > > > > with additional definitions and as such I came up with a macro that has > > > > > to be used in .c file and that will define appropriate inline func. > > > > > > > > > > I'm definitely looking for better names for REFCOUNT_RELEASE_TAKE_USE_ > > > > > macro, assuming it has to stay. > > > > > > > > You could shorten it to REFCNT_REL_TAKE_ > > > > > > > > > > All function use full 'refcount_release' and the like, so that would be > > > inconsistent. > > > > > > Losing 'take' may be an option, I don't know. > > > > Yeh, the only advantage is that it only appears once per file used, > > so it's not THAT long... > > > > > > > Comments? > > > > > > > > > > > Will you update the refcount(9) man page w/ documentation before > > > > committing? > > > > > > Sure. > > > > Thanks. -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."
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