[PATCH RFC cmpxchg 8/8] riscv: Emulate one-byte and two-byte cmpxchg

[Paul E. McKenney]

On Thu, Apr 04, 2024 at 07:15:40AM -0700, Palmer Dabbelt wrote:
> On Mon, 01 Apr 2024 14:39:50 PDT (-0700), paulmck at kernel.org wrote:
> > Use the new cmpxchg_emu_u8() and cmpxchg_emu_u16() to emulate one-byte
> > and two-byte cmpxchg() on riscv.
> > 
> > [ paulmck: Apply kernel test robot feedback. ]
> 
> I'm not entirely following the thread, but sounds like there's going to be
> generic kernel users of this now?  Before we'd said "no" to the byte/half
> atomic emulation routines beacuse they weren't used, but if it's a generic
> thing then I'm find adding them.

RCU currently contains an open-coded counterpart of the proposed
cmpxchg_emu_u8() function, so yes.  ;-)

> There's a patch set over here
> <https://lore.kernel.org/all/20240103163203.72768-2-leobras@redhat.com/>
> that implements these more directly using LR/SC.  I was sort of on the fence
> about just taking it even with no direct users right now, as the byte/half
> atomic extension is working its way through the spec process so we'll have
> them for real soon.  I stopped right there for the last merge window,
> though, as I figured it was too late to be messing with the atomics...

I would be extremely happy to drop my riscv patch in favor of an
architecture-specific implementation, especially a more-efficient
implementation.  ;-)

> So
> 
> Acked-by: Palmer Dabbelt <palmer at rivosinc.com>
> 
> if you guys want to take some sort of tree-wide change to make the byte/half
> stuff be required everywhere.  We'll eventually end up with arch routines
> for the extension, so at that point we might as well also have the more
> direct LR/SC flavors.
> 
> If you want I can go review/merge that RISC-V patch set and then it'll have
> time to bake for a shared tag you can pick up for all this stuff?  No rush
> on my end, just LMK.

That sounds very good!  I will apply your ack to my emulatino commit
in the meantime, so your schedule is my schedule.  And a big "thank
you!" for both!!!

							Thanx, Paul

> > Signed-off-by: Paul E. McKenney <paulmck at kernel.org>
> > Cc: Andi Shyti <andi.shyti at linux.intel.com>
> > Cc: Andrzej Hajda <andrzej.hajda at intel.com>
> > Cc: <linux-riscv at lists.infradead.org>
> > ---
> >  arch/riscv/Kconfig               |  1 +
> >  arch/riscv/include/asm/cmpxchg.h | 25 +++++++++++++++++++++++++
> >  2 files changed, 26 insertions(+)
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index be09c8836d56b..4eaf40d0a52ec 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -44,6 +44,7 @@ config RISCV
> >  	select ARCH_HAS_UBSAN
> >  	select ARCH_HAS_VDSO_DATA
> >  	select ARCH_KEEP_MEMBLOCK if ACPI
> > +	select ARCH_NEED_CMPXCHG_1_2_EMU
> >  	select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
> >  	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
> >  	select ARCH_STACKWALK
> > diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> > index 2fee65cc84432..a5b377481785c 100644
> > --- a/arch/riscv/include/asm/cmpxchg.h
> > +++ b/arch/riscv/include/asm/cmpxchg.h
> > @@ -9,6 +9,7 @@
> >  #include <linux/bug.h>
> > 
> >  #include <asm/fence.h>
> > +#include <linux/cmpxchg-emu.h>
> > 
> >  #define __xchg_relaxed(ptr, new, size)					\
> >  ({									\
> > @@ -170,6 +171,12 @@
> >  	__typeof__(*(ptr)) __ret;					\
> >  	register unsigned int __rc;					\
> >  	switch (size) {							\
> > +	case 1:								\
> > +		__ret = cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
> > +		break;							\
> > +	case 2:								\
> > +		break;							\
> > +		__ret = cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
> >  	case 4:								\
> >  		__asm__ __volatile__ (					\
> >  			"0:	lr.w %0, %2\n"				\
> > @@ -214,6 +221,12 @@
> >  	__typeof__(*(ptr)) __ret;					\
> >  	register unsigned int __rc;					\
> >  	switch (size) {							\
> > +	case 1:								\
> > +		__ret = cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
> > +		break;							\
> > +	case 2:								\
> > +		break;							\
> > +		__ret = cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
> >  	case 4:								\
> >  		__asm__ __volatile__ (					\
> >  			"0:	lr.w %0, %2\n"				\
> > @@ -260,6 +273,12 @@
> >  	__typeof__(*(ptr)) __ret;					\
> >  	register unsigned int __rc;					\
> >  	switch (size) {							\
> > +	case 1:								\
> > +		__ret = cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
> > +		break;							\
> > +	case 2:								\
> > +		break;							\
> > +		__ret = cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
> >  	case 4:								\
> >  		__asm__ __volatile__ (					\
> >  			RISCV_RELEASE_BARRIER				\
> > @@ -306,6 +325,12 @@
> >  	__typeof__(*(ptr)) __ret;					\
> >  	register unsigned int __rc;					\
> >  	switch (size) {							\
> > +	case 1:								\
> > +		__ret = cmpxchg_emu_u8((volatile u8 *)__ptr, __old, __new); \
> > +		break;							\
> > +	case 2:								\
> > +		break;							\
> > +		__ret = cmpxchg_emu_u16((volatile u16 *)__ptr, __old, __new); \
> >  	case 4:								\
> >  		__asm__ __volatile__ (					\
> >  			"0:	lr.w %0, %2\n"				\