ACCESS_ONCE does not work reliably on non-scalar types. For example gcc 4.6 and 4.7 might remove the volatile tag for such accesses during the SRA (scalar replacement of aggregates) steps. see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 This patch is based on an initial proof-of-concept from Linus Torvalds that causes compile errors for such accesses. Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx> --- include/linux/compiler.h | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/include/linux/compiler.h b/include/linux/compiler.h index d5ad7b1..8a92c93 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -377,8 +377,18 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); * 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. See the x86 + * spinlock implementation as an example. For other cases like page table + * structures, a barrier might be a good alternative. */ -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +#define get_scalar_volatile_pointer(x) ({ \ + typeof(x) *__p = &(x); \ + volatile typeof(x) *__vp = __p; \ + (void)(long)*__p; __vp; }) +#define ACCESS_ONCE(x) (*get_scalar_volatile_pointer(x)) /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */ #ifdef CONFIG_KPROBES -- 1.9.3
