Re: [RFC PATCH v3 4/5] riscv/cmpxchg: Implement cmpxchg for variables of size 1 and 2

From: Leonardo Bras Soares Passos
Date: Tue Aug 08 2023 - 22:03:58 EST

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Hello Guo Ren,

On Mon, Aug 7, 2023 at 1:17 PM Leonardo Bras Soares Passos
<leobras@xxxxxxxxxx> wrote:
>
> 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" \

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.

It was actually the problem :)
Even though I revised a lot, I was missing this which was made clear
by test-running on a VM:

"0: lr.w %0, %2\n" \
" and %0, %0, %z5\n" \
" bne %0, %z3, 1f\n" \
" and %1, %0, %z6\n" \
" or %1, %1, %z4\n" \
" sc.w" sc_sfx " %1, %1, %2\n" \
" bnez %1, 0b\n"

If I do "and %0, %0, %z5" there, the line "and %1, %0, %z6" will
output %1 = 0, which is not the intended.
You were right! The above fix solves the issue, and I just have to
mask it after.

Sorry it took me a few days to realize this.

> >
> > 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.

changed type to ulong, no changes detected in asm

>
> >
> > 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.
>

Some changes found (for xchg16):
- pointer alignment wrong (need mask with ~0x3) : solved. Doesn't seem
to need to be volatile, though.

- AND with 0x3 for getting shift: I was trusting 2-byte vars to be
2-byte aligned , ie. p & 0x1 == 0: may not be true. Solved (it's
simple)

- on your code, IIUC you also assume ptr & 0x1 == 0, on
ulong *__ptr = (ulong *)((ulong)ptr & ~2);
if it's not aligned, you may have __ptr = 0xYYYYYY01
(Not saying it's wrong to assume this, just listing It differs on my code)

- also verified your code generates addw and sllw on some points mine
generates add and sll, don't see any issue there

- There are extra sll and srl in my asm, because I end up casting to
typeof(*ptr) at the end.

changes found in cmpxchg (__cmpxchg_small_relaxed)
- mostly the same as above,

- the beginning of my function ends up being smaller, since I do my
masking differently.

I fixed the above issues, and ran some userspace debugging tests on a
VM, and all seems to be correct now.

I then fetched your github repo, replaced my patches v3->v4 on your
tree, added linux boot parameter "qspinlock" on my qemu cmdline
(-machine virt): booting and working just fine.

I will send a v4 soon, please give it a test when possible.

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
> >

Next message: Kemeng Shi: "[PATCH 0/2] Two minor cleanups for pcp list in page_alloc"
Previous message: Furong Xu: "[PATCH net-next v3 1/1] net: stmmac: xgmac: RX queue routing configuration"
Next in thread: Leonardo Bras Soares Passos: "Re: [RFC PATCH v3 4/5] riscv/cmpxchg: Implement cmpxchg for variables of size 1 and 2"
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