Re: [RFC PATCH v3 4/5] riscv/cmpxchg: Implement cmpxchg for variables of size 1 and 2
From: Leonardo Bras Soares Passos
Date: Mon Aug 07 2023 - 12:18:35 EST
On Sat, Aug 5, 2023 at 1:24 AM Guo Ren <guoren@xxxxxxxxxx> wrote:
>
> On Sat, Aug 5, 2023 at 11:14 AM Leonardo Bras Soares Passos
> <leobras@xxxxxxxxxx> wrote:
> >
> > Hello Guo Ren, thanks for the feedback!
> >
> > On Fri, Aug 4, 2023 at 2:45 PM Guo Ren <guoren@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Aug 4, 2023 at 4:49 AM Leonardo Bras <leobras@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > > 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?
> I missed your removing "__ret & mask" at the end. So this may not the problem.
>
> Your patch can't boot. After chewing your code for several hours, I
> found a problem:
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index 943f094375c7..67bcce63b267 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -14,6 +14,7 @@
> #define __arch_xchg_mask(prepend, append, r, p, n) \
> ({ \
> /* Depends on 2-byte variables being 2-byte aligned */ \
> + volatile ulong *__p = (ulong *)((ulong)(p) & ~0x3); \
Wow, I totally missed this one: I should not assume the processor
behavior on unaligned lr/sc.
Thanks for spotting this :)
> ulong __s = ((ulong)(p) & 0x3) * BITS_PER_BYTE; \
> ulong __mask = GENMASK(((sizeof(*p)) * BITS_PER_BYTE) - 1, 0) \
> << __s; \
> @@ -29,7 +30,7 @@
> " sc.w %1, %1, %2\n" \
> " bnez %1, 0b\n" \
> append \
> - : "=&r" (__retx), "=&r" (__rc), "+A" (*(p)) \
> + : "=&r" (__retx), "=&r" (__rc), "+A" (*(__p)) \
> : "rJ" (__newx), "rJ" (~__mask) \
> : "memory"); \
> \
> @@ -106,6 +107,7 @@
> #define __arch_cmpxchg_mask(sc_sfx, prepend, append, r, p, o, n) \
> ({ \
> /* Depends on 2-byte variables being 2-byte aligned */ \
> + volatile ulong *__p = (ulong *)((ulong)(p) & ~0x3); \
> ulong __s = ((ulong)(p) & 0x3) * BITS_PER_BYTE; \
> ulong __mask = GENMASK(((sizeof(*p)) * BITS_PER_BYTE) - 1, 0) \
> << __s; \
> @@ -125,7 +127,7 @@
> " bnez %1, 0b\n" \
> append \
> "1:\n" \
> - : "=&r" (__retx), "=&r" (__rc), "+A" (*(p)) \
> + : "=&r" (__retx), "=&r" (__rc), "+A" (*(__p)) \
> : "rJ" ((long)__oldx), "rJ" (__newx), \
> "rJ" (__mask), "rJ" (~__mask) \
> : "memory"); \
>
> But the lkvm-static still can't boot with paravirt_spinlock .... Are
> there any atomic tests in the Linux?
I recall reading something about 'locktorture' or so, not sure if useful:
https://docs.kernel.org/locking/locktorture.html
>
> I found you use some "register int variables". Would it cause the problem?
Honestly, not sure.
I will try inspecting the generated asm for any unexpected changes
compared to your version.
>
> You can reference this file, and it has passed the lock torture test:
> https://github.com/guoren83/linux/blob/sg2042-master-qspinlock-64ilp32_v4/arch/riscv/include/asm/cmpxchg.h
>
> I also merged your patches with the qspinlock series: (Use the above
> cmpxchg.h the lkvm would run normally.)
> https://github.com/guoren83/linux/tree/qspinlock_v11
Thanks!
I should reply as soon as I find anything.
Best regards,
Leo
>
>
>
> >
> > > > + 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
> > >
> >
>
>
> --
> Best Regards
> Guo Ren
>