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

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 <[email protected]>
> > Cc: Vineet Gupta <[email protected]>
> > Cc: Andi Shyti <[email protected]>
> > Cc: Andrzej Hajda <[email protected]>
> > Cc: Arnd Bergmann <[email protected]>
> > Cc: Palmer Dabbelt <[email protected]>
> > Cc: <[email protected]>
> > ---
> >  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 <[email protected]>
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 <[email protected]>
    Cc: Vineet Gupta <[email protected]>
    Cc: Andi Shyti <[email protected]>
    Cc: Andrzej Hajda <[email protected]>
    Cc: Arnd Bergmann <[email protected]>
    Cc: Palmer Dabbelt <[email protected]>
    Cc: <[email protected]>

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	\
 	 */								\