Quoting Christian König (2017-09-11 10:06:50) > Am 11.09.2017 um 10:59 schrieb Chris Wilson: > > Quoting Christian König (2017-09-11 09:50:40) > >> Sorry for the delayed response, but your mail somehow ended up in the > >> Spam folder. > >> > >> Am 04.09.2017 um 15:40 schrieb Chris Wilson: > >>> Quoting Christian König (2017-09-04 14:27:33) > >>>> From: Christian König <christian.koe...@amd.com> > >>>> > >>>> The logic is buggy and unnecessary complex. When dma_fence_get_rcu() > >>>> fails to > >>>> acquire a reference it doesn't necessary mean that there is no fence at > >>>> all. > >>>> > >>>> It usually mean that the fence was replaced by a new one and in this > >>>> situation > >>>> we certainly want to have the new one as result and *NOT* NULL. > >>> Which is not guaranteed by the code you wrote either. > >>> > >>> The point of the comment is that the mb is only inside the successful > >>> kref_atomic_inc_unless_zero, and that only after that mb do you know > >>> whether or not you have the current fence. > >>> > >>> You can argue that you want to replace the > >>> if (!dma_fence_get_rcu()) > >>> return NULL > >>> with > >>> if (!dma_fence_get_rcu() > >>> continue; > >>> but it would be incorrect to say that by simply ignoring the > >>> post-condition check that you do have the right fence. > >> You are completely missing the point here. > >> > >> It is irrelevant if you have the current fence or not when you return. > >> You can only guarantee that it is the current fence when you take a look > >> and that is exactly what we want to avoid. > >> > >> So the existing code is complete nonsense. Instead what we need to > >> guarantee is that we return *ANY* fence which we can grab a reference for. > > Not quite. We can grab a reference on a fence that was already freed and > > reused between the rcu_dereference() and dma_fence_get_rcu(). > > Reusing a memory structure before the RCU grace period is completed is > illegal, otherwise the whole RCU approach won't work.
RCU only protects that the pointer remains valid. If you use SLAB_TYPESAFE_BY_RCU, it is possible to reuse the pointer within a grace period. It does happen and that is the point the comment is trying to make. -Chris
