On Thu, Nov 7, 2024 at 11:02 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
>
>
> On Wed, Nov 6, 2024, at 14:40, Jason Gunthorpe wrote:
> > On Wed, Nov 06, 2024 at 11:01:20AM +0100, Uros Bizjak wrote:
> >> On Wed, Nov 6, 2024 at 9:55 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >> >
> >> > On Tue, Nov 5, 2024, at 13:30, Joerg Roedel wrote:
> >> > > On Fri, Nov 01, 2024 at 04:22:57PM +0000, Suravee Suthikulpanit wrote:
> >> > >> include/asm-generic/rwonce.h | 2 +-
> >> > >> include/linux/compiler_types.h | 8 +++++++-
> >> > >> 2 files changed, 8 insertions(+), 2 deletions(-)
> >> > >
> >> > > This patch needs Cc:
> >> > >
> >> > > Arnd Bergmann <arnd@xxxxxxxx>
> >> > > linux-arch@xxxxxxxxxxxxxxx
> >> > >
> >> >
> >> > It also needs an update to the comment about why this is safe:
> >> >
> >> > >> +++ b/include/asm-generic/rwonce.h
> >> > >> @@ -33,7 +33,7 @@
> >> > >> * (e.g. a virtual address) and a strong prevailing wind.
> >> > >> */
> >> > >> #define compiletime_assert_rwonce_type(t) \
> >> > >> - compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long), \
> >> > >> + compiletime_assert(__native_word(t) || sizeof(t) == sizeof(__dword_type), \
> >> > >> "Unsupported access size for {READ,WRITE}_ONCE().")
> >> >
> >> > As far as I can tell, 128-but words don't get stored atomically on
> >> > any architecture, so this seems wrong, because it would remove
> >> > the assertion on someone incorrectly using WRITE_ONCE() on a
> >> > 128-bit variable.
> >>
> >> READ_ONCE() and WRITE_ONCE() do not guarantee atomicity for double
> >> word types. They only guarantee (c.f. include/asm/generic/rwonce.h):
> >>
> >> * Prevent the compiler from merging or refetching reads or writes. The
> >> * compiler is also forbidden from reordering successive instances of
> >> * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
> >> * particular ordering. ...
> >>
> >> and later:
> >>
> >> * Yes, this permits 64-bit accesses on 32-bit architectures. These will
> >> * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
> >> * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
> >> * (e.g. a virtual address) and a strong prevailing wind.
> >>
> >> This is the "strong prevailing wind", mentioned in the patch review at [1].
> >>
> >> [1] https://lore.kernel.org/lkml/20241016130819.GJ3559746@xxxxxxxxxx/
>
> I understand the special case for ARMv7VE. I think the more important
> comment in that file is
>
> * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
> * atomicity. Note that this may result in tears!
>
> The entire point of compiletime_assert_rwonce_type() is to ensure
> that these are accesses fit the stricter definition, and I would
> prefer to not extend that to 64-bit architecture. If there are users
> that need the "once" behavior but not require atomicity of the
> access, can't that just use __READ_ONCE() instead?
If this is the case, then the patch could be simply something like the
attached (untested) patch.
Thanks,
Uros.
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 1a957ea2f4fe..94a18e8ccf7e 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -469,7 +469,8 @@ struct ftrace_likely_data {
unsigned type: (unsigned type)0, \
signed type: (signed type)0
-#define __unqual_scalar_typeof(x) typeof( \
+#if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)
+ #define __unqual_scalar_typeof(x) typeof( \
_Generic((x), \
char: (char)0, \
__scalar_type_to_expr_cases(char), \
@@ -477,7 +478,19 @@ struct ftrace_likely_data {
__scalar_type_to_expr_cases(int), \
__scalar_type_to_expr_cases(long), \
__scalar_type_to_expr_cases(long long), \
+ __scalar_type_to_expr_cases(__int128), \
default: (x)))
+#else
+ #define __unqual_scalar_typeof(x) typeof( \
+ _Generic((x), \
+ char: (char)0, \
+ __scalar_type_to_expr_cases(char), \
+ __scalar_type_to_expr_cases(short), \
+ __scalar_type_to_expr_cases(int), \
+ __scalar_type_to_expr_cases(long), \
+ __scalar_type_to_expr_cases(long long), \
+ default: (x)))
+#endif
/* Is this type a native word size -- useful for atomic operations */
#define __native_word(t) \
