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

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Message-ID: <11c53e21-f17a-4e9b-bcea-b0a0c03bf46e@paulmck-laptop>
Date: Tue, 6 Aug 2024 06:57:32 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Vineet Gupta <vgupta@...nel.org>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	elver@...gle.com, akpm@...ux-foundation.org, tglx@...utronix.de,
	peterz@...radead.org, torvalds@...ux-foundation.org, arnd@...db.de,
	geert@...ux-m68k.org, palmer@...osinc.com, mhiramat@...nel.org,
	linux-sh@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
	Andi Shyti <andi.shyti@...ux.intel.com>,
	Andrzej Hajda <andrzej.hajda@...el.com>
Subject: Re: [PATCH cmpxchg 2/3] ARC: Emulate one-byte cmpxchg

On Mon, Aug 05, 2024 at 09:44:39PM -0700, Vineet Gupta wrote:
> 
> 
> 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@...nel.org>
> >>> Cc: Vineet Gupta <vgupta@...nel.org>
> >>> Cc: Andi Shyti <andi.shyti@...ux.intel.com>
> >>> Cc: Andrzej Hajda <andrzej.hajda@...el.com>
> >>> Cc: Arnd Bergmann <arnd@...db.de>
> >>> Cc: Palmer Dabbelt <palmer@...osinc.com>
> >>> Cc: <linux-snps-arc@...ts.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@...nel.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@...nel.org>
> >     Cc: Vineet Gupta <vgupta@...nel.org>
> >     Cc: Andi Shyti <andi.shyti@...ux.intel.com>
> >     Cc: Andrzej Hajda <andrzej.hajda@...el.com>
> >     Cc: Arnd Bergmann <arnd@...db.de>
> >     Cc: Palmer Dabbelt <palmer@...osinc.com>
> >     Cc: <linux-snps-arc@...ts.infradead.org>
> 
> Acked-by: Vineet Gupta <vgupta@...nel.org>

Thank you, and I will apply this on my next rebase.

							Thanx, Paul

> 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	\
> >  	 */								\
> 
>