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kernel: tighten rules for ACCESS ONCE

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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@de.ibm.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Christian Borntraeger 2014-11-25 10:16:39 +01:00
parent 38c5ce936a
commit 927609d622
1 changed files with 16 additions and 5 deletions
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21
include/linux/compiler.h
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@ -447,12 +447,23 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
* 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