Re: [PATCH v1 1/5] riscv/cmpxchg: Deduplicate xchg() asm functions

Boqun Feng
2024-01-04T21:52:31Z

On Thu, Jan 04, 2024 at 05:41:26PM -0300, Leonardo Bras wrote:
> On Thu, Jan 04, 2024 at 11:53:45AM -0800, Boqun Feng wrote:
> > On Wed, Jan 03, 2024 at 01:31:59PM -0300, Leonardo Bras wrote:
> > > In this header every xchg define (_relaxed, _acquire, _release, vanilla)
> > > contain it's own asm file, both for 4-byte variables an 8-byte variables,
> > > on a total of 8 versions of mostly the same asm.
> > >
> > > This is usually bad, as it means any change may be done in up to 8
> > > different places.
> > >
> > > Unify those versions by creating a new define with enough parameters to
> > > generate any version of the previous 8.
> > >
> > > Then unify the result under a more general define, and simplify
> > > arch_xchg* generation.
> > >
> > > (This did not cause any change in generated asm)
> > >
> > > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > > Reviewed-by: Guo Ren <guoren@kernel.org>
> > > Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
> > > Tested-by: Guo Ren <guoren@kernel.org>
> > > ---
> > > arch/riscv/include/asm/cmpxchg.h | 138 ++++++-------------------------
> > > 1 file changed, 23 insertions(+), 115 deletions(-)
> > >
> > > diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> > > index 2f4726d3cfcc2..48478a8eecee7 100644
> > > --- a/arch/riscv/include/asm/cmpxchg.h
> > > +++ b/arch/riscv/include/asm/cmpxchg.h
> > > @@ -11,140 +11,48 @@
> > > #include <asm/barrier.h>
> > > #include <asm/fence.h>
> > >
> > > -#define __xchg_relaxed(ptr, new, size) \
> > > +#define __arch_xchg(sfx, prepend, append, r, p, n) \
> > > ({ \
> > > - __typeof__(ptr) __ptr = (ptr); \
> > > - __typeof__(new) __new = (new); \
> > > - __typeof__(*(ptr)) __ret; \
> > > - switch (size) { \
> > > - case 4: \
> > > - __asm__ __volatile__ ( \
> > > - " amoswap.w %0, %2, %1\n" \
> > > - : "=r" (__ret), "+A" (*__ptr) \
> > > - : "r" (__new) \
> > > - : "memory"); \
>
> Hello Boqun, thanks for reviewing!
>
> >
> > Hmm... actually xchg_relaxed() doesn't need to be a barrier(), so the
> > "memory" clobber here is not needed here. Of course, it's out of the
> > scope of this series, but I'm curious to see what would happen if we
> > remove the "memory" clobber _relaxed() ;-)
>
> Nice question :)
> I am happy my patch can help bring up those ideas :)
>
>
> According to gcc.gnu.org:
>
> ---
> "memory" [clobber]:
>
> The "memory" clobber tells the compiler that the assembly code
> performs memory reads or writes to items other than those listed in
> the input and output operands (for example, accessing the memory
> pointed to by one of the input parameters). To ensure memory contains

Note here it says "other than those listed in the input and output
operands", and in the above asm block, the memory pointed by "__ptr" is
already marked as read-and-write by the asm block via "+A" (*__ptr), so
the compiler knows the asm block may modify the memory pointed by
"__ptr", therefore in _relaxed() case, "memory" clobber can be avoided.

Here is an example showing the difference, considering the follow case:

this_val = *this;
that_val = *that;
xchg_relaxed(this, 1);
reread_this = *this;

by the semantics of _relaxed, compilers can optimize the above into

this_val = *this;
xchg_relaxed(this, 1);
that_val = *that;
reread_this = *this;

but the "memory" clobber in the xchg_relexed() will provide this.
Needless to say the '"+A" (*__ptr)' prevents compiler from the following
optimization:

this_val = *this;
that_val = *that;
xchg_relaxed(this, 1);
reread_this = this_val;

since the compiler knows the asm block will read and write *this.

Regards,
Boqun

> correct values, GCC may need to flush specific register values to
> memory before executing the asm. Further, the compiler does not assume
> that any values read from memory before an asm remain unchanged after
> that asm ; it reloads them as needed. Using the "memory" clobber
> effectively forms a read/write memory barrier for the compiler.
>
> Note that this clobber does not prevent the processor from doing
> speculative reads past the asm statement. To prevent that, you need
> processor-specific fence instructions.
> ---
>
> IIUC above text says that having memory accesses to *__ptr would require
> above asm to have the "memory" clobber, so memory accesses don't get
> reordered by the compiler.
>
> By above affirmation, all asm in this file should have the "memory"
> clobber, since all atomic operations will change memory pointed by an input
> ptr. Is that correct?
>
> Thanks!
> Leo
>
>
> >
> > Regards,
> > Boqun
> >
> > > - break; \
> > > - case 8: \
> > > - __asm__ __volatile__ ( \
> > > - " amoswap.d %0, %2, %1\n" \
> > > - : "=r" (__ret), "+A" (*__ptr) \
> > > - : "r" (__new) \
> > > - : "memory"); \
> > > - break; \
> > > - default: \
> > > - BUILD_BUG(); \
> > > - } \
> > > - __ret; \
> > > -})
> > > -
> > > -#define arch_xchg_relaxed(ptr, x) \
> > > -({ \
> > > - __typeof__(*(ptr)) _x_ = (x); \
> > > - (__typeof__(*(ptr))) __xchg_relaxed((ptr), \
> > > - _x_, sizeof(*(ptr))); \
> > > + __asm__ __volatile__ ( \
> > > + prepend \
> > > + " amoswap" sfx " %0, %2, %1\n" \
> > > + append \
> > > + : "=r" (r), "+A" (*(p)) \
> > > + : "r" (n) \
> > > + : "memory"); \
> > > })
> > >
> > > -#define __xchg_acquire(ptr, new, size) \
> > > +#define _arch_xchg(ptr, new, sfx, prepend, append) \
> > > ({ \
> > > __typeof__(ptr) __ptr = (ptr); \
> > > - __typeof__(new) __new = (new); \
> > > - __typeof__(*(ptr)) __ret; \
> > > - switch (size) { \
> > > + __typeof__(*(__ptr)) __new = (new); \
> > > + __typeof__(*(__ptr)) __ret; \
> > > + switch (sizeof(*__ptr)) { \
> > > case 4: \
> > > - __asm__ __volatile__ ( \
> > > - " amoswap.w %0, %2, %1\n" \
> > > - RISCV_ACQUIRE_BARRIER \
> > > - : "=r" (__ret), "+A" (*__ptr) \
> > > - : "r" (__new) \
> > > - : "memory"); \
> > > + __arch_xchg(".w" sfx, prepend, append, \
> > > + __ret, __ptr, __new); \
> > > break; \
> > > case 8: \
> > > - __asm__ __volatile__ ( \
> > > - " amoswap.d %0, %2, %1\n" \
> > > - RISCV_ACQUIRE_BARRIER \
> > > - : "=r" (__ret), "+A" (*__ptr) \
> > > - : "r" (__new) \
> > > - : "memory"); \
> > > + __arch_xchg(".d" sfx, prepend, append, \
> > > + __ret, __ptr, __new); \
> > > break; \
> > > default: \
> > > BUILD_BUG(); \
> > > } \
> > > - __ret; \
> > > + (__typeof__(*(__ptr)))__ret; \
> > > })
> > >
> > > -#define arch_xchg_acquire(ptr, x) \
> > > -({ \
> > > - __typeof__(*(ptr)) _x_ = (x); \
> > > - (__typeof__(*(ptr))) __xchg_acquire((ptr), \
> > > - _x_, sizeof(*(ptr))); \
> > > -})
> > > +#define arch_xchg_relaxed(ptr, x) \
> > > + _arch_xchg(ptr, x, "", "", "")
> > >
> > > -#define __xchg_release(ptr, new, size) \
> > > -({ \
> > > - __typeof__(ptr) __ptr = (ptr); \
> > > - __typeof__(new) __new = (new); \
> > > - __typeof__(*(ptr)) __ret; \
> > > - switch (size) { \
> > > - case 4: \
> > > - __asm__ __volatile__ ( \
> > > - RISCV_RELEASE_BARRIER \
> > > - " amoswap.w %0, %2, %1\n" \
> > > - : "=r" (__ret), "+A" (*__ptr) \
> > > - : "r" (__new) \
> > > - : "memory"); \
> > > - break; \
> > > - case 8: \
> > > - __asm__ __volatile__ ( \
> > > - RISCV_RELEASE_BARRIER \
> > > - " amoswap.d %0, %2, %1\n" \
> > > - : "=r" (__ret), "+A" (*__ptr) \
> > > - : "r" (__new) \
> > > - : "memory"); \
> > > - break; \
> > > - default: \
> > > - BUILD_BUG(); \
> > > - } \
> > > - __ret; \
> > > -})
> > > +#define arch_xchg_acquire(ptr, x) \
> > > + _arch_xchg(ptr, x, "", "", RISCV_ACQUIRE_BARRIER)
> > >
> > > #define arch_xchg_release(ptr, x) \
> > > -({ \
> > > - __typeof__(*(ptr)) _x_ = (x); \
> > > - (__typeof__(*(ptr))) __xchg_release((ptr), \
> > > - _x_, sizeof(*(ptr))); \
> > > -})
> > > -
> > > -#define __arch_xchg(ptr, new, size) \
> > > -({ \
> > > - __typeof__(ptr) __ptr = (ptr); \
> > > - __typeof__(new) __new = (new); \
> > > - __typeof__(*(ptr)) __ret; \
> > > - switch (size) { \
> > > - case 4: \
> > > - __asm__ __volatile__ ( \
> > > - " amoswap.w.aqrl %0, %2, %1\n" \
> > > - : "=r" (__ret), "+A" (*__ptr) \
> > > - : "r" (__new) \
> > > - : "memory"); \
> > > - break; \
> > > - case 8: \
> > > - __asm__ __volatile__ ( \
> > > - " amoswap.d.aqrl %0, %2, %1\n" \
> > > - : "=r" (__ret), "+A" (*__ptr) \
> > > - : "r" (__new) \
> > > - : "memory"); \
> > > - break; \
> > > - default: \
> > > - BUILD_BUG(); \
> > > - } \
> > > - __ret; \
> > > -})
> > > + _arch_xchg(ptr, x, "", RISCV_RELEASE_BARRIER, "")
> > >
> > > #define arch_xchg(ptr, x) \
> > > -({ \
> > > - __typeof__(*(ptr)) _x_ = (x); \
> > > - (__typeof__(*(ptr))) __arch_xchg((ptr), _x_, sizeof(*(ptr))); \
> > > -})
> > > + _arch_xchg(ptr, x, ".aqrl", "", "")
> > >
> > > #define xchg32(ptr, x) \
> > > ({ \
> > > --
> > > 2.43.0
> > >
> >
>