Am 25.11.2014 um 16:59 schrieb Paul E. McKenney: > On Tue, Nov 25, 2014 at 01:38:29PM +0100, Christian Borntraeger wrote: >> 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) step >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145) >> >> Let's provide READ_ONCE/ASSIGN_ONCE that will do all accesses via >> scalar types. >> >> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx> >> --- >> include/linux/compiler.h | 34 ++++++++++++++++++++++++++++++++++ >> 1 file changed, 34 insertions(+) >> >> diff --git a/include/linux/compiler.h b/include/linux/compiler.h >> index d5ad7b1..0ff01f2 100644 >> --- a/include/linux/compiler.h >> +++ b/include/linux/compiler.h >> @@ -186,6 +186,40 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); >> # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__) >> #endif >> >> +#include <linux/types.h> >> + >> +static __always_inline void __assign_once_size(volatile void *p, void *res, int size) >> +{ >> + switch (size) { >> + case 1: *(volatile u8 *)p = *(u8 *)res; break; >> + case 2: *(volatile u16 *)p = *(u16 *)res; break; >> + case 4: *(volatile u32 *)p = *(u32 *)res; break; >> +#ifdef CONFIG_64BIT >> + case 8: *(volatile u64 *)p = *(u64 *)res; break; > > We really need something like this to catch invalid sizes: > > default: invoke_nonexistent_function(); > > Of course, a BUILD_BUG_ON() would give a nicer error message. > > Without this, in my testing, the following compiles without error, generating > no code: > > struct foo { > int field[10]; > } f, f1; > > f1 = READ_ONCE(f); > > There is probably some better way to do this. Yes, I was trying to do something for default, but we are in compiler.h and BUILD_BUG_ON etc are not defined. including other header files gave me some trouble, but doing it only inside the ifdef !assembly might work out (as I did with linux/types.h). The thing is this case was actually detected see the pmd_t compile error for m68k and sparc. Defining an extern function named read_once_called_for_large_object and then let the linker do the real work might work out as well. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
