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Daniel Vetter authored
The trouble with having a plain nesting flag for locks which do not naturally nest (unlike block devices and their partitions, which is the original motivation for nesting levels) is that lockdep will never spot a true deadlock if you screw up. This patch is an attempt at trying better, by highlighting a bit more of the actual nature of the nesting that's going on. Essentially we have two kinds of objects: - objects without pages allocated, which cannot be on any lru and are hence inaccessible to the shrinker. - objects which have pages allocated, which are on an lru, and which the shrinker can decide to throw out. For the former type of object, memory allocations while holding obj->mm.lock are permissible. For the latter they are not. And get/put_pages transitions between the two types of objects. This is still not entirely fool-proof since the rules might change. But as long as we run such a code ever at runtime lockdep should be able to observe the inconsistency and complain (like with any other lockdep class that we've split up in multiple classes). But there are a few clear benefits: - We can drop the nesting flag parameter from __i915_gem_object_put_pages, because that function by definition is never going allocate memory, and calling it on an object which doesn't have its pages allocated would be a bug. - We strictly catch more bugs, since there's not only one place in the entire tree which is annotated with the special class. All the other places that had explicit lockdep nesting annotations we're now going to leave up to lockdep again. - Specifically this catches stuff like calling get_pages from put_pages (which isn't really a good idea, if we can call get_pages so could the shrinker). I've seen patches do exactly that. Of course I fully expect CI will show me for the fool I am with this one here :-) v2: There can only be one (lockdep only has a cache for the first subclass, not for deeper ones, and we don't want to make these locks even slower). Still separate enums for better documentation. Real fix: don't forget about phys objs and pin_map(), and fix the shrinker to have the right annotations ... silly me. v3: Forgot usertptr too ... v4: Improve comment for pages_pin_count, drop the IMPORTANT comment and instead prime lockdep (Chris). v5: Appease checkpatch, no double empty lines (Chris) v6: More rebasing over selftest changes. Also somehow I forgot to push this patch :-/ Also format comments consistently while at it. v7: Fix typo in commit message (Joonas) Also drop the priming, with the lmem merge we now have allocations while holding the lmem lock, which wreaks the generic priming I've done in earlier patches. Should probably be resurrected when lmem is fixed. See commit 232a6eba Author: Matthew Auld <matthew.auld@intel.com> Date: Tue Oct 8 17:01:14 2019 +0100 drm/i915: introduce intel_memory_region I'm keeping the priming patch locally so it wont get lost. Cc: Matthew Auld <matthew.auld@intel.com> Cc: Chris Wilson <chris@chris-wilson.co.uk> Cc: "Tang, CQ" <cq.tang@intel.com> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v5) Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> (v6) Signed-off-by:
Daniel Vetter <daniel.vetter@intel.com> Signed-off-by:
Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by:
Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20191105090148.30269-1-daniel.vetter@ffwll.ch [mlankhorst: Fix commit typos pointed out by Michael Ruhl]
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