rcu: Move lockless_dereference() out of rcupdate.h

I want to use lockless_dereference() from seqlock.h, which would mean
including rcupdate.h from it, however rcupdate.h already includes
seqlock.h.

Avoid this by moving lockless_dereference() into compiler.h. This is
somewhat tricky since it uses smp_read_barrier_depends() which isn't
available there, but its a CPP macro so we can get away with it.

The alternative would be moving it into asm/barrier.h, but that would
be updating each arch (I can do if people feel that is more
appropriate).

Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 15 insertions, 0 deletions

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 867722591be2..eae42c21d5fd 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -457,6 +457,21 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
         (volatile typeof(x) *)&(x); })
 #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
 
+/**
+ * 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); \
+})
+
 /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
 #ifdef CONFIG_KPROBES
 # define __kprobes      __attribute__((__section__(".kprobes.text")))