Messages in this thread | | | From | Leonardo Bras Soares Passos <> | Date | Sat, 5 Aug 2023 00:14:31 -0300 | Subject | Re: [RFC PATCH v3 4/5] riscv/cmpxchg: Implement cmpxchg for variables of size 1 and 2 |
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Hello Guo Ren, thanks for the feedback!
On Fri, Aug 4, 2023 at 2:45 PM Guo Ren <guoren@kernel.org> wrote: > > On Fri, Aug 4, 2023 at 4:49 AM Leonardo Bras <leobras@redhat.com> wrote: > > > > cmpxchg for variables of size 1-byte and 2-bytes is not yet available for > > riscv, even though its present in other architectures such as arm64 and > > x86. This could lead to not being able to implement some locking mechanisms > > or requiring some rework to make it work properly. > > > > Implement 1-byte and 2-bytes cmpxchg in order to achieve parity with other > > architectures. > > > > Signed-off-by: Leonardo Bras <leobras@redhat.com> > > --- > > arch/riscv/include/asm/cmpxchg.h | 35 ++++++++++++++++++++++++++++++++ > > 1 file changed, 35 insertions(+) > > > > diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h > > index 5a07646fae65..dfb433ac544f 100644 > > --- a/arch/riscv/include/asm/cmpxchg.h > > +++ b/arch/riscv/include/asm/cmpxchg.h > > @@ -72,6 +72,36 @@ > > * indicated by comparing RETURN with OLD. > > */ > > > > +#define __arch_cmpxchg_mask(sc_sfx, prepend, append, r, p, o, n) \ > > +({ \ > > + /* Depends on 2-byte variables being 2-byte aligned */ \ > > + ulong __s = ((ulong)(p) & 0x3) * BITS_PER_BYTE; \ > > + ulong __mask = GENMASK(((sizeof(*p)) * BITS_PER_BYTE) - 1, 0) \ > > + << __s; \ > > + ulong __newx = (ulong)(n) << __s; \ > > + ulong __oldx = (ulong)(o) << __s; \ > > + ulong __retx; \ > > + register unsigned int __rc; \ > > + \ > > + __asm__ __volatile__ ( \ > > + prepend \ > > + "0: lr.w %0, %2\n" \ > > + " and %0, %0, %z5\n" \ > > + " bne %0, %z3, 1f\n" \
> bug: > - " and %0, %0, %z5\n" \ > - " bne %0, %z3, 1f\n" \ > + " and %1, %0, %z5\n" \ > + " bne %1, %z3, 1f\n" \ > Your code breaks the %0.
What do you mean by breaks here?
In the end of this macro, I intended to have __retx = (*p & __mask) which means the value is clean to be rotated at the end of the macro (no need to apply the mask again): r = __ret >> __s;
Also, I assumed we are supposed to return the same variable type as the pointer, so this is valid: u8 a, *b, c; a = xchg(b, c);
Is this correct?
> > + append \ > > + "1:\n" \ > > + : "=&r" (__retx), "=&r" (__rc), "+A" (*(p)) \ > > + : "rJ" ((long)__oldx), "rJ" (__newx), \ > > + "rJ" (__mask), "rJ" (~__mask) \ > > + : "memory"); \ > > + \ > > + r = (__typeof__(*(p)))(__retx >> __s); \ > > +}) > > + > > > > #define __arch_cmpxchg(lr_sfx, sc_sfx, prepend, append, r, p, co, o, n) \ > > ({ \ > > @@ -98,6 +128,11 @@ > > __typeof__(*(ptr)) __ret; \ > > \ > > switch (sizeof(*__ptr)) { \ > > + case 1: \ > > + case 2: \ > > + __arch_cmpxchg_mask(sc_sfx, prepend, append, \ > > + __ret, __ptr, __old, __new); \ > > + break; \ > > case 4: \ > > __arch_cmpxchg(".w", ".w" sc_sfx, prepend, append, \ > > __ret, __ptr, (long), __old, __new); \ > > -- > > 2.41.0 > > > > > -- > Best Regards > Guo Ren >
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