Re: [PATCH cmpxchg 2/3] ARC: Emulate one-byte cmpxchg

[Vineet Gupta <vgupta@kernel.org>]

On 8/5/24 21:28, Paul E. McKenney wrote:
> On Mon, Aug 05, 2024 at 06:27:57PM -0700, Vineet Gupta wrote:
>> Hi Paul,
>>
>> On 8/5/24 12:21, Paul E. McKenney wrote:
>>> Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on arc.
>>>
>>> [ paulmck: Drop two-byte support per Arnd Bergmann feedback. ]
>>> [ paulmck: Apply feedback from Naresh Kamboju. ]
>>> [ paulmck: Apply kernel test robot feedback. ]
>>>
>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>> Cc: Vineet Gupta <vgupta@kernel.org>
>>> Cc: Andi Shyti <andi.shyti@linux.intel.com>
>>> Cc: Andrzej Hajda <andrzej.hajda@intel.com>
>>> Cc: Arnd Bergmann <arnd@arndb.de>
>>> Cc: Palmer Dabbelt <palmer@rivosinc.com>
>>> Cc: <linux-snps-arc@lists.infradead.org>
>>> ---
>>>  arch/arc/Kconfig               |  1 +
>>>  arch/arc/include/asm/cmpxchg.h | 33 ++++++++++++++++++++++++---------
>>>  2 files changed, 25 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
>>> index fd0b0a0d4686a..163608fd49d18 100644
>>> --- a/arch/arc/Kconfig
>>> +++ b/arch/arc/Kconfig
>>> @@ -13,6 +13,7 @@ config ARC
>>>  	select ARCH_HAS_SETUP_DMA_OPS
>>>  	select ARCH_HAS_SYNC_DMA_FOR_CPU
>>>  	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
>>> +	select ARCH_NEED_CMPXCHG_1_EMU
>>>  	select ARCH_SUPPORTS_ATOMIC_RMW if ARC_HAS_LLSC
>>>  	select ARCH_32BIT_OFF_T
>>>  	select BUILDTIME_TABLE_SORT
>>> diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
>>> index e138fde067dea..2102ce076f28b 100644
>>> --- a/arch/arc/include/asm/cmpxchg.h
>>> +++ b/arch/arc/include/asm/cmpxchg.h
>>> @@ -8,6 +8,7 @@
>>>  
>>>  #include <linux/build_bug.h>
>>>  #include <linux/types.h>
>>> +#include <linux/cmpxchg-emu.h>
>>>  
>>>  #include <asm/barrier.h>
>>>  #include <asm/smp.h>
>>> @@ -46,6 +47,9 @@
>>>  	__typeof__(*(ptr)) _prev_;					\
>>>  									\
>>>  	switch(sizeof((_p_))) {						\
>>> +	case 1:								\
>>> +		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
>>> +		break;							\
>>>  	case 4:								\
>>>  		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
>>>  		break;							\
>>> @@ -65,16 +69,27 @@
>>>  	__typeof__(*(ptr)) _prev_;					\
>>>  	unsigned long __flags;						\
>>>  									\
>>> -	BUILD_BUG_ON(sizeof(_p_) != 4);					\
>> Is this alone not sufficient: i.e. for !LLSC let the atomic op happen
>> under a spin-lock for non 4 byte quantities as well.
> Now that you mention it, that would be a lot simpler.
>
>>> +	switch(sizeof((_p_))) {						\
>>> +	case 1:								\
>>> +		__flags = cmpxchg_emu_u8((volatile u8 *)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
>>> +		_prev_ = (__typeof__(*(ptr)))__flags;			\
>>> +		break;							\
>>> +		break;							\
>> FWIW, the 2nd break seems extraneous.
> And to your earlier point, the first break as well.  ;-)
>
> How does the updated patch below look?  Or did I miss your point?
>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 96c1107797ca329fe203818cdfda2fe5f5a9a82e
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Mon Mar 18 01:27:35 2024 -0700
>
>     ARC: Emulate one-byte cmpxchg
>     
>     Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on arc.
>     
>     [ paulmck: Drop two-byte support per Arnd Bergmann feedback. ]
>     [ paulmck: Apply feedback from Naresh Kamboju. ]
>     [ paulmck: Apply kernel test robot feedback. ]
>     [ paulmck: Apply feedback from Vineet Gupta. ]
>     
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>     Cc: Vineet Gupta <vgupta@kernel.org>
>     Cc: Andi Shyti <andi.shyti@linux.intel.com>
>     Cc: Andrzej Hajda <andrzej.hajda@intel.com>
>     Cc: Arnd Bergmann <arnd@arndb.de>
>     Cc: Palmer Dabbelt <palmer@rivosinc.com>
>     Cc: <linux-snps-arc@lists.infradead.org>

Acked-by: Vineet Gupta <vgupta@kernel.org>

Thx,
-Vineet

>
> diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
> index fd0b0a0d4686a..163608fd49d18 100644
> --- a/arch/arc/Kconfig
> +++ b/arch/arc/Kconfig
> @@ -13,6 +13,7 @@ config ARC
>  	select ARCH_HAS_SETUP_DMA_OPS
>  	select ARCH_HAS_SYNC_DMA_FOR_CPU
>  	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
> +	select ARCH_NEED_CMPXCHG_1_EMU
>  	select ARCH_SUPPORTS_ATOMIC_RMW if ARC_HAS_LLSC
>  	select ARCH_32BIT_OFF_T
>  	select BUILDTIME_TABLE_SORT
> diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
> index e138fde067dea..58045c8983404 100644
> --- a/arch/arc/include/asm/cmpxchg.h
> +++ b/arch/arc/include/asm/cmpxchg.h
> @@ -8,6 +8,7 @@
>  
>  #include <linux/build_bug.h>
>  #include <linux/types.h>
> +#include <linux/cmpxchg-emu.h>
>  
>  #include <asm/barrier.h>
>  #include <asm/smp.h>
> @@ -46,6 +47,9 @@
>  	__typeof__(*(ptr)) _prev_;					\
>  									\
>  	switch(sizeof((_p_))) {						\
> +	case 1:								\
> +		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
> +		break;							\
>  	case 4:								\
>  		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
>  		break;							\
> @@ -65,8 +69,6 @@
>  	__typeof__(*(ptr)) _prev_;					\
>  	unsigned long __flags;						\
>  									\
> -	BUILD_BUG_ON(sizeof(_p_) != 4);					\
> -									\
>  	/*								\
>  	 * spin lock/unlock provide the needed smp_mb() before/after	\
>  	 */								\