[tip:locking/core] locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()

[tip-bot for Will Deacon]

Commit-ID:  76ebbe78f7390aee075a7f3768af197ded1bdfbb
Gitweb:     https://git.kernel.org/tip/76ebbe78f7390aee075a7f3768af197ded1bdfbb
Author:     Will Deacon <will.deacon@xxxxxxx>
AuthorDate: Tue, 24 Oct 2017 11:22:47 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 24 Oct 2017 13:17:32 +0200

locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()

In preparation for the removal of lockless_dereference(), which is the
same as READ_ONCE() on all architectures other than Alpha, add an
implicit smp_read_barrier_depends() to READ_ONCE() so that it can be
used to head dependency chains on all architectures.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1508840570-22169-3-git-send-email-will.deacon@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/linux/compiler.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 0808318..7d7b77d 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -242,6 +242,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 		__read_once_size(&(x), __u.__c, sizeof(x));		\
 	else								\
 		__read_once_size_nocheck(&(x), __u.__c, sizeof(x));	\
+	smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
 	__u.__val;							\
 })
 #define READ_ONCE(x) __READ_ONCE(x, 1)