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

Paul E. McKenney
2024-08-06T04:28:59Z

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>

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 \
*/ \