Op 18-07-13 22:54, David Herrmann schreef: > Hi > > On Thu, Jul 18, 2013 at 1:24 PM, Thomas Hellstrom <thellstrom@xxxxxxxxxx> wrote: >> On 07/18/2013 01:07 PM, David Herrmann wrote: >>> Hi >>> >>> On Thu, Jul 18, 2013 at 10:53 AM, Thomas Hellstrom >>> <thellstrom@xxxxxxxxxx> wrote: >>>> A quick look, but not a full review: >>>> >>>> Looks mostly good, but it looks like the TTM vm lock isn't needed at all >>>> anymore (provided the vma offset manager is properly protected), since >>>> kref_get_unless_zero() is used when a reference after lookup is taken. >>>> (please see the kref_get_unless_zero documentation examples). When >>>> drm_vma_offset_remove() is called, the kref value is unconditionally >>>> zero, >>>> and that should block any successful lookup. >>> If we drop vm_lock without modifying TTM, this will not work. Even >>> kref_get_unless_zero() needs some guarantee that the object is still >>> valid. Assume this scenario: >>> >>> drm_vma_offset_lookup() returns a valid node, however, before we call >>> kref_get_*(), another thread destroys the last reference to the bo so >>> it gets kfree()d. kref_get_unless_zero() will thus reference memory >>> which can be _anything_ and is not guarantee to stay 0. >>> (Documentation/kref.txt mentions this, too, and I guess it was you who >>> wrote that). >>> >>> I cannot see any locking around the mmap call that could guarantee >>> this except vm_lock. >> >> You're right. My apologies. Parental leave has taken its toll. >> >> >>>> Otherwise, if the vma offset manager always needs external locking to >>>> make >>>> lookup + referencing atomic, then perhaps locking should be completely >>>> left to the caller? >>> I would actually prefer to keep it in the VMA manager. It allows some >>> fast-paths which otherwise would need special semantics for the caller >>> (for instance see the access-management patches which are based on >>> this patchset or the reduced locking during setup/release). We'd also >>> require the caller to use rwsems for good performance, which is not >>> the case for gem. >>> >>> So how about Daniel's proposal to add an _unlocked() version and >>> provide _lock_lookup() and _unlock_lookup() helpers which just wrap >>> the read_lock() and read_unlock? >>> >>> Thanks! >>> David >> >> I think that if there are good reasons to keep locking internal, I'm fine >> with that, (And also, of course, with >> Daniel's proposal). Currently the add / remove / lookup paths are mostly >> used by TTM during object creation and >> destruction. >> >> However, if the lookup path is ever used by pread / pwrite, that situation >> might change and we would then like to >> minimize the locking. > I tried to keep the change as minimal as I could. Follow-up patches > are welcome. I just thought pushing the lock into drm_vma_* would > simplify things. If there are benchmarks that prove me wrong, I'll > gladly spend some time optimizing that. > > Apart from this, I'd like to see someone ack the > ttm_buffer_object_transfer() changes. I am not very confident with > that. Everything else should be trivial. > The transfer object only exists to kill off the memory backing during asynchronous eviction, by using the delayed destroyed mechanism. The re-initialization there looks correct to me. ~Maarten _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
