On Thu, Mar 28, 2019 at 04:34:04PM -0700, John Hubbard wrote: > On 3/28/19 4:24 PM, Jerome Glisse wrote: > > On Thu, Mar 28, 2019 at 04:20:37PM -0700, John Hubbard wrote: > >> On 3/28/19 4:05 PM, Jerome Glisse wrote: > >>> On Thu, Mar 28, 2019 at 03:43:33PM -0700, John Hubbard wrote: > >>>> On 3/28/19 3:40 PM, Jerome Glisse wrote: > >>>>> On Thu, Mar 28, 2019 at 03:25:39PM -0700, John Hubbard wrote: > >>>>>> On 3/28/19 3:08 PM, Jerome Glisse wrote: > >>>>>>> On Thu, Mar 28, 2019 at 02:41:02PM -0700, John Hubbard wrote: > >>>>>>>> On 3/28/19 2:30 PM, Jerome Glisse wrote: > >>>>>>>>> On Thu, Mar 28, 2019 at 01:54:01PM -0700, John Hubbard wrote: > >>>>>>>>>> On 3/25/19 7:40 AM, jglisse@xxxxxxxxxx wrote: > >>>>>>>>>>> From: Jérôme Glisse <jglisse@xxxxxxxxxx> > >>>>>> [...] > >>>>>> OK, so let's either drop this patch, or if merge windows won't allow that, > >>>>>> then *eventually* drop this patch. And instead, put in a hmm_sanity_check() > >>>>>> that does the same checks. > >>>>> > >>>>> RDMA depends on this, so does the nouveau patchset that convert to new API. > >>>>> So i do not see reason to drop this. They are user for this they are posted > >>>>> and i hope i explained properly the benefit. > >>>>> > >>>>> It is a common pattern. Yes it only save couple lines of code but down the > >>>>> road i will also help for people working on the mmap_sem patchset. > >>>>> > >>>> > >>>> It *adds* a couple of lines that are misleading, because they look like they > >>>> make things safer, but they don't actually do so. > >>> > >>> It is not about safety, sorry if it confused you but there is nothing about > >>> safety here, i can add a big fat comment that explains that there is no safety > >>> here. The intention is to allow the page fault handler that potential have > >>> hundred of page fault queue up to abort as soon as it sees that it is pointless > >>> to keep faulting on a dying process. > >>> > >>> Again if we race it is _fine_ nothing bad will happen, we are just doing use- > >>> less work that gonna be thrown on the floor and we are just slowing down the > >>> process tear down. > >>> > >> > >> In addition to a comment, how about naming this thing to indicate the above > >> intention? I have a really hard time with this odd down_read() wrapper, which > >> allows code to proceed without really getting a lock. It's just too wrong-looking. > >> If it were instead named: > >> > >> hmm_is_exiting() > > > > What about: hmm_lock_mmap_if_alive() ? > > > > That's definitely better, but I want to vote for just doing a check, not > taking any locks. > > I'm not super concerned about the exact name, but I really want a routine that > just checks (and optionally asserts, via WARN or BUG), and that's *all*. Then > drivers can scatter that around like pixie dust as they see fit. Maybe right before > taking a lock, maybe in other places. Decoupled from locking. I agree. Names matter and any function which is called *_down_read and could potentially not take the lock should be called try_*_down_read. Furthermore users should be checking the return values from any try_*. It is also odd that we are calling "down/up" on something which is not a semaphore. So the user here needs to _know_ that they are really getting the lock on the mm which sits behind the scenes. What John is proposing is more explicit when reading driver code. Ira > > thanks, > -- > John Hubbard > NVIDIA >
