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authorChris Wilson <chris@chris-wilson.co.uk>2019-11-22 12:49:24 +0300
committerChris Wilson <chris@chris-wilson.co.uk>2019-11-22 13:47:38 +0300
commit67a3acaab7167157fb827595019eaf55df244824 (patch)
tree7c86954c4d3321faf888ae74b0f00c623905b4c7 /drivers/gpu/drm/i915/i915_scheduler.h
parentf05bfce3341208de93e4fd08bdb635dc671afec4 (diff)
downloadlinux-67a3acaab7167157fb827595019eaf55df244824.tar.xz
drm/i915: Use a ctor for TYPESAFE_BY_RCU i915_request
As we start peeking into requests for longer and longer, e.g. incorporating use of spinlocks when only protected by an rcu_read_lock(), we need to be careful in how we reset the request when recycling and need to preserve any barriers that may still be in use as the request is reset for reuse. Quoting Linus Torvalds: > If there is refcounting going on then why use SLAB_TYPESAFE_BY_RCU? .. because the object can be accessed (by RCU) after the refcount has gone down to zero, and the thing has been released. That's the whole and only point of SLAB_TYPESAFE_BY_RCU. That flag basically says: "I may end up accessing this object *after* it has been free'd, because there may be RCU lookups in flight" This has nothing to do with constructors. It's ok if the object gets reused as an object of the same type and does *not* get re-initialized, because we're perfectly fine seeing old stale data. What it guarantees is that the slab isn't shared with any other kind of object, _and_ that the underlying pages are free'd after an RCU quiescent period (so the pages aren't shared with another kind of object either during an RCU walk). And it doesn't necessarily have to have a constructor, because the thing that a RCU walk will care about is (a) guaranteed to be an object that *has* been on some RCU list (so it's not a "new" object) (b) the RCU walk needs to have logic to verify that it's still the *same* object and hasn't been re-used as something else. In contrast, a SLAB_TYPESAFE_BY_RCU memory gets free'd and re-used immediately, but because it gets reused as the same kind of object, the RCU walker can "know" what parts have meaning for re-use, in a way it couidn't if the re-use was random. That said, it *is* subtle, and people should be careful. > So the re-use might initialize the fields lazily, not necessarily using a ctor. If you have a well-defined refcount, and use "atomic_inc_not_zero()" to guard the speculative RCU access section, and use "atomic_dec_and_test()" in the freeing section, then you should be safe wrt new allocations. If you have a completely new allocation that has "random stale content", you know that it cannot be on the RCU list, so there is no speculative access that can ever see that random content. So the only case you need to worry about is a re-use allocation, and you know that the refcount will start out as zero even if you don't have a constructor. So you can think of the refcount itself as always having a zero constructor, *BUT* you need to be careful with ordering. In particular, whoever does the allocation needs to then set the refcount to a non-zero value *after* it has initialized all the other fields. And in particular, it needs to make sure that it uses the proper memory ordering to do so. NOTE! One thing to be very worried about is that re-initializing whatever RCU lists means that now the RCU walker may be walking on the wrong list so the walker may do the right thing for this particular entry, but it may miss walking *other* entries. So then you can get spurious lookup failures, because the RCU walker never walked all the way to the end of the right list. That ends up being a much more subtle bug. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20191122094924.629690-1-chris@chris-wilson.co.uk
Diffstat (limited to 'drivers/gpu/drm/i915/i915_scheduler.h')
-rw-r--r--drivers/gpu/drm/i915/i915_scheduler.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/drivers/gpu/drm/i915/i915_scheduler.h b/drivers/gpu/drm/i915/i915_scheduler.h
index 07d243acf553..d1dc4efef77b 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.h
+++ b/drivers/gpu/drm/i915/i915_scheduler.h
@@ -26,6 +26,7 @@
sched.link)
void i915_sched_node_init(struct i915_sched_node *node);
+void i915_sched_node_reinit(struct i915_sched_node *node);
bool __i915_sched_node_add_dependency(struct i915_sched_node *node,
struct i915_sched_node *signal,