linux-kernel - Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch

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Date:   Tue, 14 Apr 2020 01:19:06 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
CC:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_"
 causes kernel crash

On 8/30/18 7:43 AM, Peter Zijlstra wrote:
> On Thu, Aug 30, 2018 at 04:29:20PM +0200, Peter Zijlstra wrote:
>
>> Also, once it all works, they should look at switching to _relaxed
>> atomics for LL/SC.
> A little something like so.. should save a few smp_mb().

Finally got to this - time for some spring cleaning ;-)


> ---
>
> diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
> index 4e0072730241..714b54c308b0 100644
> --- a/arch/arc/include/asm/atomic.h
> +++ b/arch/arc/include/asm/atomic.h
> @@ -44,7 +44,7 @@ static inline void atomic_##op(int i, atomic_t *v)			\
>  }									\
>  
>  #define ATOMIC_OP_RETURN(op, c_op, asm_op)				\
> -static inline int atomic_##op##_return(int i, atomic_t *v)		\
> +static inline int atomic_##op##_return_relaxed(int i, atomic_t *v)	\
>  {									\

This being relaxed, shoudn't it also remove the smp_mb() before the operation and
leave the generic code to add one / more smp_mb() as appropriate for fully
ordered, acquire and release variants ?

>  	unsigned int val;						\
>  									\
> @@ -69,8 +69,11 @@ static inline int atomic_##op##_return(int i, atomic_t *v)		\
>  	return val;							\
>  }
>  
> +#define atomic_add_return_relaxed	atomic_add_return_relaxed
> +#define atomic_sub_return_relaxed	atomic_sub_return_relaxed
> +
>  #define ATOMIC_FETCH_OP(op, c_op, asm_op)				\
> -static inline int atomic_fetch_##op(int i, atomic_t *v)			\
> +static inline int atomic_fetch_##op##_relaxed(int i, atomic_t *v)	\
>  {									\
>  	unsigned int val, orig;						\
>  									\
> @@ -96,6 +99,14 @@ static inline int atomic_fetch_##op(int i, atomic_t *v)			\
>  	return orig;							\
>  }
>  
> +#define atomic_fetch_add_relaxed	atomic_fetch_add_relaxed
> +#define atomic_fetch_sub_relaxed	atomic_fetch_sub_relaxed
> +
> +#define atomic_fetch_and_relaxed	atomic_fetch_and_relaxed
> +#define atomic_fetch_andnot_relaxed	atomic_fetch_andnot_relaxed
> +#define atomic_fetch_or_relaxed		atomic_fetch_or_relaxed
> +#define atomic_fetch_xor_relaxed	atomic_fetch_xor_relaxed
> +
>  #else	/* !CONFIG_ARC_HAS_LLSC */
>  
>  #ifndef CONFIG_SMP
> @@ -379,7 +390,7 @@ static inline void atomic64_##op(long long a, atomic64_t *v)		\
>  }									\
>  
>  #define ATOMIC64_OP_RETURN(op, op1, op2)		        	\
> -static inline long long atomic64_##op##_return(long long a, atomic64_t *v)	\
> +static inline long long atomic64_##op##_return_relaxed(long long a, atomic64_t *v)	\
>  {									\
>  	unsigned long long val;						\
>  									\
> @@ -401,8 +412,11 @@ static inline long long atomic64_##op##_return(long long a, atomic64_t *v)	\
>  	return val;							\
>  }
>  
> +#define atomic64_add_return_relaxed	atomic64_add_return_relaxed
> +#define atomic64_sub_return_relaxed	atomic64_sub_return_relaxed
> +
>  #define ATOMIC64_FETCH_OP(op, op1, op2)		        		\
> -static inline long long atomic64_fetch_##op(long long a, atomic64_t *v)	\
> +static inline long long atomic64_fetch_##op##_relaxed(long long a, atomic64_t *v)	\
>  {									\
>  	unsigned long long val, orig;					\
>  									\
> @@ -424,6 +438,14 @@ static inline long long atomic64_fetch_##op(long long a, atomic64_t *v)	\
>  	return orig;							\
>  }
>  
> +#define atomic64_fetch_add_relaxed	atomic64_fetch_add_relaxed
> +#define atomic64_fetch_sub_relaxed	atomic64_fetch_sub_relaxed
> +
> +#define atomic64_fetch_and_relaxed	atomic64_fetch_and_relaxed
> +#define atomic64_fetch_andnot_relaxed	atomic64_fetch_andnot_relaxed
> +#define atomic64_fetch_or_relaxed	atomic64_fetch_or_relaxed
> +#define atomic64_fetch_xor_relaxed	atomic64_fetch_xor_relaxed
> +
>  #define ATOMIC64_OPS(op, op1, op2)					\
>  	ATOMIC64_OP(op, op1, op2)					\
>  	ATOMIC64_OP_RETURN(op, op1, op2)				\
> @@ -434,6 +456,12 @@ static inline long long atomic64_fetch_##op(long long a, atomic64_t *v)	\
>  
>  ATOMIC64_OPS(add, add.f, adc)
>  ATOMIC64_OPS(sub, sub.f, sbc)
> +
> +#undef ATOMIC64_OPS
> +#define ATOMIC64_OPS(op, op1, op2)					\
> +	ATOMIC64_OP(op, op1, op2)					\
> +	ATOMIC64_FETCH_OP(op, op1, op2)
> +

For clarity I split off this hunk into a seperate patch as it elides generation of
unused bitwise ops.

>  ATOMIC64_OPS(and, and, and)
>  ATOMIC64_OPS(andnot, bic, bic)
>  ATOMIC64_OPS(or, or, or)
>