On 03/21/2018 11:03 AM, Jerome Glisse wrote: > On Tue, Mar 20, 2018 at 09:14:34PM -0700, John Hubbard wrote: >> On 03/19/2018 07:00 PM, jgli...@redhat.com wrote: >>> From: Ralph Campbell <rcampb...@nvidia.com>
<snip> >> Hi Jerome, >> >> This presents a deadlock problem (details below). As for solution ideas, >> Mark Hairgrove points out that the MMU notifiers had to solve the >> same sort of problem, and part of the solution involves "avoid >> holding locks when issuing these callbacks". That's not an entire >> solution description, of course, but it seems like a good start. >> >> Anyway, for the deadlock problem: >> >> Each of these ->release callbacks potentially has to wait for the >> hmm_invalidate_range() callbacks to finish. That is not shown in any >> code directly, but it's because: when a device driver is processing >> the above ->release callback, it has to allow any in-progress operations >> to finish up (as specified clearly in your comment documentation above). >> >> Some of those operations will invariably need to do things that result >> in page invalidations, thus triggering the hmm_invalidate_range() callback. >> Then, the hmm_invalidate_range() callback tries to acquire the same >> hmm->mirrors_sem lock, thus leading to deadlock: >> >> hmm_invalidate_range(): >> // ... >> down_read(&hmm->mirrors_sem); >> list_for_each_entry(mirror, &hmm->mirrors, list) >> mirror->ops->sync_cpu_device_pagetables(mirror, action, >> start, end); >> up_read(&hmm->mirrors_sem); > > That is just illegal, the release callback is not allowed to trigger > invalidation all it does is kill all device's threads and stop device > page fault from happening. So there is no deadlock issues. I can re- > inforce the comment some more (see [1] for example on what it should > be). That rule is fine, and it is true that the .release callback will not directly trigger any invalidations. However, the problem is in letting any *existing* outstanding operations finish up. We have to let existing operations "drain", in order to meet the requirement that everything is done when .release returns. For example, if a device driver thread is in the middle of working through its fault buffer, it will call migrate_vma(), which will in turn unmap pages. That will cause an hmm_invalidate_range() callback, which tries to take hmm->mirrors_sems, and we deadlock. There's no way to "kill" such a thread while it's in the middle of migrate_vma(), you have to let it finish up. > > Also it is illegal for the sync callback to trigger any mmu_notifier > callback. I thought this was obvious. The sync callback should only > update device page table and do _nothing else_. No way to make this > re-entrant. That is obvious, yes. I am not trying to say there is any problem with that rule. It's the "drain outstanding operations during .release", above, that is the real problem. thanks, -- John Hubbard NVIDIA > > For anonymous private memory migrated to device memory it is freed > shortly after the release callback (see exit_mmap()). For share memory > you might want to migrate back to regular memory but that will be fine > as you will not get mmu_notifier callback any more. > > So i don't see any deadlock here. > > Cheers, > Jérôme > > [1] > https://cgit.freedesktop.org/~glisse/linux/commit/?h=nouveau-hmm&id=93adb3e6b4f39d5d146b6a8afb4175d37bdd4890 >
