In the debate as to whether the second read of active->request is
ordered after the dependent reads of the first read of active->request,
just give in and throw a smp_rmb() in there so that ordering of loads is
assured.
v2: Explain the manual smp_rmb()
Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vet...@ffwll.ch>
---
drivers/gpu/drm/i915/i915_gem.c | 25 ++++++++++++++++++++-----
drivers/gpu/drm/i915/i915_gem_request.h | 3 +++
2 files changed, 23 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index f4f8eaa90f2a..654f0b015f97 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3735,7 +3735,7 @@ i915_gem_object_ggtt_unpin_view(struct
drm_i915_gem_object *obj,
i915_vma_unpin(i915_gem_obj_to_ggtt_view(obj, view));
}
-static __always_inline unsigned __busy_read_flag(unsigned int id)
+static __always_inline unsigned int __busy_read_flag(unsigned int id)
{
/* Note that we could alias engines in the execbuf API, but
* that would be very unwise as it prevents userspace from
@@ -3753,7 +3753,7 @@ static __always_inline unsigned int
__busy_write_id(unsigned int id)
return id;
}
-static __always_inline unsigned
+static __always_inline unsigned int
__busy_set_if_active(const struct i915_gem_active *active,
unsigned int (*flag)(unsigned int id))
{
@@ -3770,19 +3770,34 @@ __busy_set_if_active(const struct i915_gem_active
*active,
id = request->engine->exec_id;
- /* Check that the pointer wasn't reassigned and overwritten. */
+ /* Check that the pointer wasn't reassigned and overwritten.
+ *
+ * In __i915_gem_active_get_rcu(), we enforce ordering between
+ * the first rcu pointer dereference (imposing a
+ * read-dependency only on access through the pointer) and
+ * the second lockless access through the memory barrier
+ * following a successful atomic_inc_not_zero(). Here there
+ * is no such barrier, and so we must manually insert an
+ * explicit read barrier to ensure that the following
+ * access occurs after all the loads through the first
+ * pointer.
+ *
+ * The corresponding write barrier is part of
+ * rcu_assign_pointer().
+ */
+ smp_rmb();
if (request == rcu_access_pointer(active->request))
return flag(id);
} while (1);
}
-static inline unsigned
+static __always_inline unsigned int
busy_check_reader(const struct i915_gem_active *active)
{
return __busy_set_if_active(active, __busy_read_flag);
}
-static inline unsigned
+static __always_inline unsigned int
busy_check_writer(const struct i915_gem_active *active)
{
return __busy_set_if_active(active, __busy_write_id);
diff --git a/drivers/gpu/drm/i915/i915_gem_request.h
b/drivers/gpu/drm/i915/i915_gem_request.h
index 3496e28785e7..b2456dede3ad 100644
--- a/drivers/gpu/drm/i915/i915_gem_request.h
+++ b/drivers/gpu/drm/i915/i915_gem_request.h
@@ -497,6 +497,9 @@ __i915_gem_active_get_rcu(const struct i915_gem_active
*active)
* incremented) then the following read for rcu_access_pointer()
* must occur after the atomic operation and so confirm
* that this request is the one currently being tracked.
+ *
+ * The corresponding write barrier is part of
+ * rcu_assign_pointer().
*/
if (!request || request == rcu_access_pointer(active->request))
return rcu_pointer_handoff(request);
--
2.8.1
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