linux-kernel - Re: [PATCH] MIPS: Compare __SYNC_loongson3

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Message-ID: <CAKwvOdmiWhtr6qkKDFnVXqLZhyFU4bfhjTyQ6BKiLYup+qasMQ@mail.gmail.com>
Date:   Thu, 14 Jan 2021 09:48:31 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        LKML <linux-kernel@...r.kernel.org>, linux-mips@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] MIPS: Compare __SYNC_loongson3_war against 0

On Thu, Jan 14, 2021 at 9:34 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> When building with clang when CONFIG_CPU_LOONGSON3_WORKAROUNDS is
> enabled:
>
>  In file included from lib/errseq.c:4:
>  In file included from ./include/linux/atomic.h:7:
>  ./arch/mips/include/asm/atomic.h:52:1: warning: converting the result of
>  '<<' to a boolean always evaluates to true
>  [-Wtautological-constant-compare]
>  ATOMIC_OPS(atomic64, s64)
>  ^
>  ./arch/mips/include/asm/atomic.h:40:9: note: expanded from macro
>  'ATOMIC_OPS'
>          return cmpxchg(&v->counter, o, n);
>                 ^
>  ./arch/mips/include/asm/cmpxchg.h:194:7: note: expanded from macro
>  'cmpxchg'
>          if (!__SYNC_loongson3_war)
>               ^
>  ./arch/mips/include/asm/sync.h:147:34: note: expanded from macro
>  '__SYNC_loongson3_war'
>  # define __SYNC_loongson3_war   (1 << 31)
>                                     ^
>
> While it is not wrong that the result of this shift is always true in a
> boolean context, it is not a problem here. Regardless, the warning is
> really noisy so rather than making the shift a boolean implicitly, use
> it in an equality comparison so the shift is used as an integer value.
>
> Fixes: 4d1dbfe6cbec ("MIPS: atomic: Emit Loongson3 sync workarounds within asm")
> Fixes: a91f2a1dba44 ("MIPS: cmpxchg: Omit redundant barriers for Loongson3")
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

Acked-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  arch/mips/include/asm/atomic.h  | 2 +-
>  arch/mips/include/asm/cmpxchg.h | 6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/mips/include/asm/atomic.h b/arch/mips/include/asm/atomic.h
> index f904084fcb1f..27ad76791539 100644
> --- a/arch/mips/include/asm/atomic.h
> +++ b/arch/mips/include/asm/atomic.h
> @@ -248,7 +248,7 @@ static __inline__ int pfx##_sub_if_positive(type i, pfx##_t * v)    \
>          * bltz that can branch to code outside of the LL/SC loop. As   \
>          * such, we don't need to emit another barrier here.            \
>          */                                                             \
> -       if (!__SYNC_loongson3_war)                                      \
> +       if (__SYNC_loongson3_war == 0)                                  \
>                 smp_mb__after_atomic();                                 \
>                                                                         \
>         return result;                                                  \
> diff --git a/arch/mips/include/asm/cmpxchg.h b/arch/mips/include/asm/cmpxchg.h
> index 5b0b3a6777ea..ed8f3f3c4304 100644
> --- a/arch/mips/include/asm/cmpxchg.h
> +++ b/arch/mips/include/asm/cmpxchg.h
> @@ -99,7 +99,7 @@ unsigned long __xchg(volatile void *ptr, unsigned long x, int size)
>          * contains a completion barrier prior to the LL, so we don't   \
>          * need to emit an extra one here.                              \
>          */                                                             \
> -       if (!__SYNC_loongson3_war)                                      \
> +       if (__SYNC_loongson3_war == 0)                                  \
>                 smp_mb__before_llsc();                                  \
>                                                                         \
>         __res = (__typeof__(*(ptr)))                                    \
> @@ -191,7 +191,7 @@ unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
>          * contains a completion barrier prior to the LL, so we don't   \
>          * need to emit an extra one here.                              \
>          */                                                             \
> -       if (!__SYNC_loongson3_war)                                      \
> +       if (__SYNC_loongson3_war == 0)                                  \
>                 smp_mb__before_llsc();                                  \
>                                                                         \
>         __res = cmpxchg_local((ptr), (old), (new));                     \
> @@ -201,7 +201,7 @@ unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
>          * contains a completion barrier after the SC, so we don't      \
>          * need to emit an extra one here.                              \
>          */                                                             \
> -       if (!__SYNC_loongson3_war)                                      \
> +       if (__SYNC_loongson3_war == 0)                                  \
>                 smp_llsc_mb();                                          \
>                                                                         \
>         __res;                                                          \
>
> base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837

^ is this "base-commit" something you manually added, or can git
format generate that?

-- 
Thanks,
~Nick Desaulniers