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. > > Note that the above rule means that for any given struct hmm_mirror their > can only be one and only one call to hmm_mirror_register() happening, no > concurrent call. If you are doing the latter then something is seriously > wrong in your design. > > So to be clear on what variable are you claiming race ? > mirror->hmm ? > mirror->hmm->mm which is really hmm->mm (mirror part does not matter) ? > > I will hold resending v4 until tomorrow morning (eastern time) so that > you can convince yourself that this code is right or prove me wrong. No need to wait. The documentation request above is a minor point, and we're OK with you resending v4 whenever you're ready. thanks, -- John Hubbard NVIDIA
