linux-kernel - [PATCH v2 2/5] locking/barriers: Add implicit smp_read_barrier_depends() to READ

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Message-Id: <1508840570-22169-3-git-send-email-will.deacon@arm.com>
Date:   Tue, 24 Oct 2017 11:22:47 +0100
From:   Will Deacon <will.deacon@....com>
To:     mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH v2 2/5] 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.

Cc: Ingo Molnar <mingo@...nel.org>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Will Deacon <will.deacon@....com>
---
 include/linux/compiler.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 08083186e54f..7d7b77da9716 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)
-- 
2.1.4