From: Wei Wang <tracyw...@gmail.com> Date: Fri, 16 Jun 2017 10:47:23 -0700
> From: Wei Wang <wei...@google.com> > > The current mechanism of dst release is a bit complicated. It is because > the users of dst get divided into 2 situations: > 1. Most users take the reference count when using a dst and release the > reference count when done. > 2. Exceptional users like IPv4/IPv6/decnet/xfrm routing code do not take > reference count when referencing to a dst due to some histotic reasons. > > Due to those exceptional use cases in 2, reference count being 0 is not an > adequate evidence to indicate that no user is using this dst. So users in 1 > can't free the dst simply based on reference count being 0 because users in > 2 might still hold reference to it. > Instead, a dst garbage list is needed to hold the dst entries that already > get removed by the users in 2 but are still held by users in 1. And a periodic > garbage collector task is run to check all the dst entries in the list to see > if the users in 1 have released the reference to those dst entries. > If so, the dst is now ready to be freed. > > This logic introduces unnecessary complications in the dst code which makes it > hard to understand and to debug. > > In order to get rid of the whole dst garbage collector (gc) and make the dst > code more unified and simplified, we can make the users in 2 also take > reference > count on the dst and release it properly when done. > This way, dst can be safely freed once the refcount drops to 0 and no gc > thread is needed anymore. > > This patch series' target is to completely get rid of dst gc logic and free > dst based on reference count only. > Patch 1-3 are preparation patches to do some cleanup/improvement on the > existing > code to make later work easier. > Patch 4-21 are real implementations. > In these patches, a temporary flag DST_NOGC is used to help transition > those exceptional users one by one. Once every component is transitioned, > this temporary flag is removed. > By the end of this patch series, all dst are refcounted when being used > and released when done. And dst will be freed when its refcount drops to 0. > No dst gc task is running anymore. > > Note: This patch series depends on the decnet fix that was sent right before: > "decnet: always not take dst->__refcnt when inserting dst into hash > table" Other than the minor feedback I gave, this series looks great! Indeed the code is a lot simpler afterwards and much easier to audit and understand. I'll wait a bit for some others to give feedback as well.
