drm/i915: Add smp_rmb() to busy ioctl's RCU dance
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 <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470731014-6894-1-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index bb83069..373136f 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3733,7 +3733,7 @@
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
@@ -3751,7 +3751,7 @@
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))
{
@@ -3768,19 +3768,45 @@
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.
+ *
+ * It is worth comparing this sequence with
+ * raw_write_seqcount_latch() which operates very similarly.
+ * The challenge here is the visibility of the other CPU
+ * writes to the reallocated request vs the local CPU ordering.
+ * Before the other CPU can overwrite the request, it will
+ * have updated our active->request and gone through a wmb.
+ * During the read here, we want to make sure that the values
+ * we see have not been overwritten as we do so - and we do
+ * that by serialising the second pointer check with the writes
+ * on other other CPUs.
+ *
+ * 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);