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Tighten rules for ACCESS_ONCE

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This series tightens the rules for ACCESS_ONCE to only work
 on scalar types. It also contains the necessary fixups as
 indicated by build bots of linux-next.
 Now everything is in place to prevent new non-scalar users
 of ACCESS_ONCE and we can continue to convert code to
 READ_ONCE/WRITE_ONCE.
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Merge tag 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/borntraeger/linux

Pull ACCESS_ONCE() rule tightening from Christian Borntraeger:
 "Tighten rules for ACCESS_ONCE

  This series tightens the rules for ACCESS_ONCE to only work on scalar
  types.  It also contains the necessary fixups as indicated by build
  bots of linux-next.  Now everything is in place to prevent new
  non-scalar users of ACCESS_ONCE and we can continue to convert code to
  READ_ONCE/WRITE_ONCE"

* tag 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/borntraeger/linux:
  kernel: Fix sparse warning for ACCESS_ONCE
  next: sh: Fix compile error
  kernel: tighten rules for ACCESS ONCE
  mm/gup: Replace ACCESS_ONCE with READ_ONCE
  x86/spinlock: Leftover conversion ACCESS_ONCE->READ_ONCE
  x86/xen/p2m: Replace ACCESS_ONCE with READ_ONCE
  ppc/hugetlbfs: Replace ACCESS_ONCE with READ_ONCE
  ppc/kvm: Replace ACCESS_ONCE with READ_ONCE
This commit is contained in:
Linus Torvalds 2015-02-14 10:54:28 -08:00
commit c833e17e27
8 changed files with 35 additions and 24 deletions
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21
include/linux/compiler.h
View file

@ -451,12 +451,23 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
* 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 = (__force typeof(x)) 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