This is a note to let you know that I've just added the patch titled
kernel: tighten rules for ACCESS ONCE
to the 3.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kernel-tighten-rules-for-access-once.patch
and it can be found in the queue-3.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 927609d622a3773995f84bc03b4564f873cf0e22 Mon Sep 17 00:00:00 2001
From: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Date: Tue, 25 Nov 2014 10:16:39 +0100
Subject: kernel: tighten rules for ACCESS ONCE
From: Christian Borntraeger <borntraeger@xxxxxxxxxx>
commit 927609d622a3773995f84bc03b4564f873cf0e22 upstream.
Now that all non-scalar users of ACCESS_ONCE have been converted
to READ_ONCE or ASSIGN once, lets tighten ACCESS_ONCE to only
work on scalar types.
This variant was proposed by Alexei Starovoitov.
Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
include/linux/compiler.h | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -447,12 +447,23 @@ static __always_inline void __write_once
* to make the compiler aware of ordering is to put the two invocations of
* ACCESS_ONCE() in different C statements.
*
- * This macro does absolutely -nothing- to prevent the CPU from reordering,
- * merging, or refetching absolutely anything at any time. Its main intended
- * use is to mediate communication between process-level code and irq/NMI
- * handlers, all running on the same CPU.
+ * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
+ * on a union member will work as long as the size of the member matches the
+ * size of the union and the size is smaller than word size.
+ *
+ * The major use cases of ACCESS_ONCE used to be (1) Mediating communication
+ * between process-level code and irq/NMI handlers, all running on the same CPU,
+ * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
+ * mutilate accesses that either do not require ordering or that interact
+ * with an explicit memory barrier or atomic instruction that provides the
+ * required ordering.
+ *
+ * If possible use READ_ONCE/ASSIGN_ONCE instead.
*/
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define __ACCESS_ONCE(x) ({ \
+ __maybe_unused typeof(x) __var = 0; \
+ (volatile typeof(x) *)&(x); })
+#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
/* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
#ifdef CONFIG_KPROBES
Patches currently in stable-queue which might be from borntraeger@xxxxxxxxxx are
queue-3.19/x86-spinlocks-paravirt-fix-memory-corruption-on-unlock.patch
queue-3.19/kernel-make-read_once-valid-on-const-arguments.patch
queue-3.19/kvm-s390-base-hrtimer-on-a-monotonic-clock.patch
queue-3.19/kernel-tighten-rules-for-access-once.patch
queue-3.19/kvm-s390-floating-irqs-fix-user-triggerable-endless-loop.patch
queue-3.19/kvm-s390-forward-hrtimer-if-guest-ckc-not-pending-yet.patch
queue-3.19/kernel-fix-sparse-warning-for-access_once.patch
queue-3.19/kvm-s390-avoid-memory-leaks-if-__inject_vm-fails.patch
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