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. > +#endif > + } > +} > + > +#define ASSIGN_ONCE(val, p) \ > + ({ typeof(p) __val; __val = val; __assign_once_size(&p, &__val, sizeof(__val)); __val; }) > + > + > +static __always_inline void __read_once_size(volatile void *p, void *res, int size) > +{ > + switch (size) { > + case 1: *(u8 *)res = *(volatile u8 *)p; break; > + case 2: *(u16 *)res = *(volatile u16 *)p; break; > + case 4: *(u32 *)res = *(volatile u32 *)p; break; > +#ifdef CONFIG_64BIT > + case 8: *(u64 *)res = *(volatile u64 *)p; break; > +#endif Ditto here. Thanx, Paul > + } > +} > + > +#define READ_ONCE(p) \ > + ({ typeof(p) __val; __read_once_size(&p, &__val, sizeof(__val)); __val; }) > + > + > #endif /* __KERNEL__ */ > > #endif /* __ASSEMBLY__ */ > -- > 1.9.3 > -- 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
