On Thu, Mar 22, 2018 at 05:13:14PM -0700, John Hubbard wrote: > On 03/22/2018 04:37 PM, Jerome Glisse wrote: > > On Thu, Mar 22, 2018 at 03:47:16PM -0700, John Hubbard wrote: > >> On 03/21/2018 04:41 PM, Jerome Glisse wrote: > >>> On Wed, Mar 21, 2018 at 04:22:49PM -0700, John Hubbard wrote: > >>>> On 03/21/2018 11:16 AM, jglisse@xxxxxxxxxx wrote: > >>>>> From: Jérôme Glisse <jglisse@xxxxxxxxxx> > > <snip> > > >>> > >>> No this code is correct. hmm->mm is set after hmm struct is allocated > >>> and before it is public so no one can race with that. It is clear in > >>> hmm_mirror_unregister() under the write lock hence checking it here > >>> under that same lock is correct. > >> > >> Are you implying that code that calls hmm_mirror_register() should do > >> it's own locking, to prevent simultaneous calls to that function? Because > >> as things are right now, multiple threads can arrive at this point. The > >> fact that mirror->hmm is not "public" is irrelevant; what matters is that > >>> 1 thread can change it simultaneously. > > > > The content of struct hmm_mirror should not be modified by code outside > > HMM after hmm_mirror_register() and before hmm_mirror_unregister(). This > > is a private structure to HMM and the driver should not touch it, ie it > > should be considered as read only/const from driver code point of view. > > Yes, that point is clear and obvious. > > > > > It is also expected (which was obvious to me) that driver only call once > > and only once hmm_mirror_register(), and only once hmm_mirror_unregister() > > for any given hmm_mirror struct. Note that driver can register multiple > > _different_ mirror struct to same mm or differents mm. > > > > There is no need of locking on the driver side whatsoever as long as the > > above rules are respected. I am puzzle if they were not obvious :) > > Those rules were not obvious. It's unusual to claim that register and unregister > can run concurrently, but regiser and register cannot. Let's please document > the rules a bit in the comments. I am really surprise this was not obvious. All existing _register API in the kernel follow this. You register something once only and doing it twice for same structure (ie unique struct hmm_mirror *mirror pointer value) leads to serious bugs (doing so concurently or not). For instance if you call mmu_notifier_register() twice (concurrently or not) with same pointer value for struct mmu_notifier *mn then bad thing will happen. Same for driver_register() but this one actualy have sanity check and complain loudly if that happens. I doubt there is any single *_register/unregister() in the kernel that does not follow this. Note that doing register/unregister concurrently for the same unique hmm_mirror struct is also illegal. However concurrent register and unregister of different hmm_mirror struct is legal and this is the reasons for races we were discussing. Cheers, Jérôme
