On Fri, Dec 13, 2019 at 11:47:06AM +0100, Luc Van Oostenryck wrote: > On Thu, Dec 12, 2019 at 09:53:38PM +0100, Peter Zijlstra wrote: > > Now, looking at the current GCC source: > > > > https://github.com/gcc-mirror/gcc/blob/97d7270f894395e513667a031a0c309d1819d05e/gcc/c/c-parser.c#L3707 > > > > it seems that __typeof__() is supposed to strip all qualifiers from > > _Atomic types. That lead me to try: > > > > typeof(_Atomic typeof(p)) __p = (p); > > > > But alas, I still get the same junk you got for ool_store_release() :/ > > I was checking this to see if Sparse was ready to support this. > I was a bit surprised because at first sigth GCC was doing as > it claims (typeof striping const & volatile on _Atomic types) > but your exampe wasn't working. But it's working if an > intermediate var is used: > _Atomic typeof(p) tmp; > typeof(tmp) __p = (p); > or, uglier but probably more practical: > typeof(({_Atomic typeof(p) tmp; })) __p = (p); > > Go figure! Excellent! I had to change it to something like: #define unqual_typeof(x) typeof(({_Atomic typeof(x) ___x __maybe_unused; ___x; })) but that does indeed work! Now I suppose we should wrap that in a symbol that indicates our compiler does indeed support _Atomic, otherwise things will come apart. That is, my gcc-4.6 doesn't seem to have it, while gcc-4.8 does, which is exactly the range that needs the daft READ_ONCE() construct, how convenient :/ Something a little like this perhaps? --- diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h index 7d9cc5ec4971..c389af602da8 100644 --- a/arch/arm64/include/asm/barrier.h +++ b/arch/arm64/include/asm/barrier.h @@ -75,9 +75,9 @@ static inline unsigned long array_index_mask_nospec(unsigned long idx, #define __smp_store_release(p, v) \ do { \ - typeof(p) __p = (p); \ - union { typeof(*p) __val; char __c[1]; } __u = \ - { .__val = (__force typeof(*p)) (v) }; \ + unqual_typeof(p) __p = (p); \ + union { unqual_typeof(*p) __val; char __c[1]; } __u = \ + { .__val = (__force unqual_typeof(*p)) (v) }; \ compiletime_assert_atomic_type(*p); \ kasan_check_write(__p, sizeof(*p)); \ switch (sizeof(*p)) { \ @@ -110,8 +110,8 @@ do { \ #define __smp_load_acquire(p) \ ({ \ - union { typeof(*p) __val; char __c[1]; } __u; \ - typeof(p) __p = (p); \ + union { unqual_typeof(*p) __val; char __c[1]; } __u; \ + unqual_typeof(p) __p = (p); \ compiletime_assert_atomic_type(*p); \ kasan_check_read(__p, sizeof(*p)); \ switch (sizeof(*p)) { \ @@ -141,8 +141,8 @@ do { \ #define smp_cond_load_relaxed(ptr, cond_expr) \ ({ \ - typeof(ptr) __PTR = (ptr); \ - typeof(*ptr) VAL; \ + unqual_typeof(ptr) __PTR = (ptr); \ + unqual_typeof(*ptr) VAL; \ for (;;) { \ VAL = READ_ONCE(*__PTR); \ if (cond_expr) \ @@ -154,8 +154,8 @@ do { \ #define smp_cond_load_acquire(ptr, cond_expr) \ ({ \ - typeof(ptr) __PTR = (ptr); \ - typeof(*ptr) VAL; \ + unqual_typeof(ptr) __PTR = (ptr); \ + unqual_typeof(*ptr) VAL; \ for (;;) { \ VAL = smp_load_acquire(__PTR); \ if (cond_expr) \ diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h index 85b28eb80b11..dd5bb055f5ab 100644 --- a/include/asm-generic/barrier.h +++ b/include/asm-generic/barrier.h @@ -228,8 +228,8 @@ do { \ */ #ifndef smp_cond_load_relaxed #define smp_cond_load_relaxed(ptr, cond_expr) ({ \ - typeof(ptr) __PTR = (ptr); \ - typeof(*ptr) VAL; \ + unqual_typeof(ptr) __PTR = (ptr); \ + unqual_typeof(*ptr) VAL; \ for (;;) { \ VAL = READ_ONCE(*__PTR); \ if (cond_expr) \ @@ -250,7 +250,7 @@ do { \ */ #ifndef smp_cond_load_acquire #define smp_cond_load_acquire(ptr, cond_expr) ({ \ - typeof(*ptr) _val; \ + unqual_typeof(*ptr) _val; \ _val = smp_cond_load_relaxed(ptr, cond_expr); \ smp_acquire__after_ctrl_dep(); \ _val; \ diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h index 0eb2a1cc411d..15fd7ea3882a 100644 --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h @@ -179,3 +179,10 @@ #endif #define __no_fgcse __attribute__((optimize("-fno-gcse"))) + +#if GCC_VERSION < 40800 +/* + * GCC-4.6 doesn't support _Atomic, which is required to strip qualifiers. + */ +#define unqual_typeof(x) typeof(x) +#endif diff --git a/include/linux/compiler.h b/include/linux/compiler.h index ad8c76144a3c..9736993f2ba1 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -279,7 +279,7 @@ void __write_once_size(volatile void *p, void *res, int size) #define __READ_ONCE(x, check) \ ({ \ - union { typeof(x) __val; char __c[1]; } __u; \ + union { unqual_typeof(x) __val; char __c[1]; } __u; \ if (check) \ __read_once_size(&(x), __u.__c, sizeof(x)); \ else \ @@ -302,12 +302,12 @@ unsigned long read_word_at_a_time(const void *addr) return *(unsigned long *)addr; } -#define WRITE_ONCE(x, val) \ -({ \ - union { typeof(x) __val; char __c[1]; } __u = \ - { .__val = (__force typeof(x)) (val) }; \ - __write_once_size(&(x), __u.__c, sizeof(x)); \ - __u.__val; \ +#define WRITE_ONCE(x, val) \ +({ \ + union { unqual_typeof(x) __val; char __c[1]; } __u = \ + { .__val = (__force unqual_typeof(x)) (val) }; \ + __write_once_size(&(x), __u.__c, sizeof(x)); \ + __u.__val; \ }) #include <linux/kcsan.h> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 72393a8c1a6c..fe8012c54251 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -243,4 +243,11 @@ struct ftrace_likely_data { #define __diag_error(compiler, version, option, comment) \ __diag_ ## compiler(version, error, option) +#ifndef unqual_typeof +/* + * GCC __typeof__() strips all qualifiers from _Atomic types. + */ +#define unqual_typeof(x) typeof(({_Atomic typeof(x) ___x __maybe_unused; ___x; })) +#endif + #endif /* __LINUX_COMPILER_TYPES_H */
