• Daniel Vetter's avatar
    drm/i915: Switch obj->mm.lock lockdep annotations on its head · f86dbacb
    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: default avatarDaniel Vetter <daniel.vetter@intel.com>
    Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
    Signed-off-by: default avatarMaarten 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]
    f86dbacb
i915_gem_object.h 13.3 KB