Re: [PATCH v2 2/7] x86: use long long for 64-bit atomic ops

[Dmitry Vyukov]

On Mon, May 29, 2017 at 4:44 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Sun, May 28, 2017 at 11:34 AM,  <hpa@xxxxxxxxx> wrote:
>> On May 28, 2017 2:29:32 AM PDT, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>>On Sun, May 28, 2017 at 1:02 AM,  <hpa@xxxxxxxxx> wrote:
>>>> On May 26, 2017 12:09:04 PM PDT, Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>>wrote:
>>>>>Some 64-bit atomic operations use 'long long' as operand/return type
>>>>>(e.g. asm-generic/atomic64.h, arch/x86/include/asm/atomic64_32.h);
>>>>>while others use 'long' (e.g. arch/x86/include/asm/atomic64_64.h).
>>>>>This makes it impossible to write portable code.
>>>>>For example, there is no format specifier that prints result of
>>>>>atomic64_read() without warnings. atomic64_try_cmpxchg() is almost
>>>>>impossible to use in portable fashion because it requires either
>>>>>'long *' or 'long long *' as argument depending on arch.
>>>>>
>>>>>Switch arch/x86/include/asm/atomic64_64.h to 'long long'.
>>>>>
>>>>>Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>>>>Cc: Mark Rutland <mark.rutland@xxxxxxx>
>>>>>Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>>>>Cc: Will Deacon <will.deacon@xxxxxxx>
>>>>>Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>>>>Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>>>>>Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>>>>>Cc: kasan-dev@xxxxxxxxxxxxxxxx
>>>>>Cc: linux-mm@xxxxxxxxx
>>>>>Cc: linux-kernel@xxxxxxxxxxxxxxx
>>>>>Cc: x86@xxxxxxxxxx
>>>>>
>>>>>---
>>>>>Changes since v1:
>>>>> - reverted stray s/long/long long/ replace in comment
>>>>> - added arch/s390 changes to fix build errors/warnings
>>>>>---
>
> [snip]
>
>>>> NAK - this is what u64/s64 is for.

Mailed v3 with all requested changes.


>>>Hi,
>>>
>>>Patch 3 adds atomic-instrumented.h which now contains:
>>>
>>>+static __always_inline long long atomic64_read(const atomic64_t *v)
>>>+{
>>>+       return arch_atomic64_read(v);
>>>+}
>>>
>>>without this patch that will become
>>>
>>>+static __always_inline s64 atomic64_read(const atomic64_t *v)
>>>
>>>Right?
>>
>> Yes.
>
>
> I see that s64 is not the same as long on x86_64 (long long vs long),
> so it's still not possible to e.g. portably print a result of
> atomic_read(). But it's a separate issue that we don't need to solve
> now.
>
> Also all wrappers like:
>
> void atomic64_set(atomic64_t *v, s64 i)
> {
>     arch_atomic64_set(v, i);
> }
>
> lead to type conversions, but at least my compiler does not bark on it.
>
> The only remaining problem is with atomic64_try_cmpxchg, which is
> simply not possible to use now (not possible to declare the *old
> type).
> I will need something along the following lines to fix it (then
> callers can use s64* for old).
> Sounds good?
>
>
> --- a/arch/x86/include/asm/atomic64_64.h
> +++ b/arch/x86/include/asm/atomic64_64.h
> @@ -177,7 +177,7 @@ static inline long
> arch_atomic64_cmpxchg(atomic64_t *v, long old, long new)
>  }
>
>  #define arch_atomic64_try_cmpxchg arch_atomic64_try_cmpxchg
> -static __always_inline bool arch_atomic64_try_cmpxchg(atomic64_t *v,
> long *old, long new)
> +static __always_inline bool arch_atomic64_try_cmpxchg(atomic64_t *v,
> s64 *old, long new)
>  {
>         return try_cmpxchg(&v->counter, old, new);
>  }
> @@ -198,7 +198,7 @@ static inline long arch_atomic64_xchg(atomic64_t
> *v, long new)
>   */
>  static inline bool arch_atomic64_add_unless(atomic64_t *v, long a, long u)
>  {
> -       long c = arch_atomic64_read(v);
> +       s64 c = arch_atomic64_read(v);
>         do {
>                 if (unlikely(c == u))
>                         return false;
> @@ -217,7 +217,7 @@ static inline bool
> arch_atomic64_add_unless(atomic64_t *v, long a, long u)
>   */
>  static inline long arch_atomic64_dec_if_positive(atomic64_t *v)
>  {
> -       long dec, c = arch_atomic64_read(v);
> +       s64 dec, c = arch_atomic64_read(v);
>         do {
>                 dec = c - 1;
>                 if (unlikely(dec < 0))
> @@ -236,7 +236,7 @@ static inline void arch_atomic64_and(long i, atomic64_t *v)
>
>  static inline long arch_atomic64_fetch_and(long i, atomic64_t *v)
>  {
> -       long val = arch_atomic64_read(v);
> +       s64 val = arch_atomic64_read(v);
>
>         do {
>         } while (!arch_atomic64_try_cmpxchg(v, &val, val & i));
> @@ -253,7 +253,7 @@ static inline void arch_atomic64_or(long i, atomic64_t *v)
>
>  static inline long arch_atomic64_fetch_or(long i, atomic64_t *v)
>  {
> -       long val = arch_atomic64_read(v);
> +       s64 val = arch_atomic64_read(v);
>
>         do {
>         } while (!arch_atomic64_try_cmpxchg(v, &val, val | i));
> @@ -270,7 +270,7 @@ static inline void arch_atomic64_xor(long i, atomic64_t *v)
>
>  static inline long arch_atomic64_fetch_xor(long i, atomic64_t *v)
>  {
> -       long val = arch_atomic64_read(v);
> +       s64 val = arch_atomic64_read(v);
>
>         do {
>         } while (!arch_atomic64_try_cmpxchg(v, &val, val ^ i));
> diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
> index e8cf95908fe5..9e2faa85eb02 100644
> --- a/arch/x86/include/asm/cmpxchg.h
> +++ b/arch/x86/include/asm/cmpxchg.h
> @@ -157,7 +157,7 @@ extern void __add_wrong_size(void)
>  #define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock)               \
>  ({                                                                     \
>         bool success;                                                   \
> -       __typeof__(_ptr) _old = (_pold);                                \
> +       __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold);              \
>         __typeof__(*(_ptr)) __old = *_old;                              \
>         __typeof__(*(_ptr)) __new = (_new);                             \
>         switch (size) {                                                 \