On Tue, Oct 28, 2014 at 01:12:14AM +0000, Al Viro wrote:
> On Mon, Oct 27, 2014 at 10:36:21AM -0700, Paul E. McKenney wrote:
> > Code making direct use of smp_read_barrier_depends() is harder to read,
> > in my experience, but good point on the sparse noise. Maybe a new
> > lockless_dereference() primitive? Maybe something like the following?
> > (Untested, probably does not even build.)
> >
> > #define lockless_dereference(p) \
> > ({ \
> > typeof(*p) *_________p1 = ACCESS_ONCE(p); \
> > smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
> > _________p1; \
> > })
>
> Hmm... Where would you prefer to put it? rcupdate.h?
Good a place as any, I guess. Please see patch below. Left to myself,
I would send this along for the next merge window, but please let me
know if you would like it sooner.
Thanx, Paul
------------------------------------------------------------------------
rcu: Provide counterpart to rcu_dereference() for non-RCU situations
Although rcu_dereference() and friends can be used in situations where
object lifetimes are being managed by something other than RCU, the
resulting sparse and lockdep-RCU noise can be annoying. This commit
therefore supplies a lockless_dereference(), which provides the
protection for dereferences without the RCU-related debugging noise.
Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9575c2d403b5..ed4f5939a452 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -617,6 +617,21 @@ static inline void rcu_preempt_sleep_check(void)
#define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v)
/**
+ * lockless_dereference() - safely load a pointer for later dereference
+ * @p: The pointer to load
+ *
+ * Similar to rcu_dereference(), but for situations where the pointed-to
+ * object's lifetime is managed by something other than RCU. That
+ * "something other" might be reference counting or simple immortality.
+ */
+#define lockless_dereference(p) \
+({ \
+ typeof(p) _________p1 = ACCESS_ONCE(p); \
+ smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
+ (_________p1); \
+})
+
+/**
* rcu_assign_pointer() - assign to RCU-protected pointer
* @p: pointer to assign to
* @v: value to assign (publish)
--
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