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

Vineet Gupta
2024-08-06T04:44:48Z

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 \
> */ \