Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

[Guo Ren <guoren@kernel.org>]

On Mon, Mar 29, 2021 at 9:56 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Mon, Mar 29, 2021 at 2:52 PM Guo Ren <guoren@kernel.org> wrote:
> >
> > On Mon, Mar 29, 2021 at 7:31 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Mon, Mar 29, 2021 at 01:16:53PM +0200, Peter Zijlstra wrote:
> > > > Anyway, an additional 'funny' is that I suspect you cannot prove fwd
> > > > progress of the entire primitive with any of this on. But who cares
> > > > about details anyway.. :/
> > >
> > > What's the architectural guarantee on LL/SC progress for RISC-V ?
> >
> > funct5    | aq | rl   | rs2 |  rs1  | funct3 | rd | opcode
> >      5          1    1      5       5         3        5          7
> > LR.W/D  ordering  0     addr    width   dest    AMO
> > SC.W/D  ordering  src  addr    width   dest    AMO
> >
> > LR.W loads a word from the address in rs1, places the sign-extended
> > value in rd, and registers a reservation set—a set of bytes that
> > subsumes the bytes in the addressed word. SC.W conditionally writes a
> > word in rs2 to the address in rs1: the SC.W succeeds only if the
> > reservation is still valid and the reservation set contains the bytes
> > being written. If the SC.W succeeds, the instruction writes the word
> > in rs2 to memory, and it writes zero to rd. If the SC.W fails, the
> > instruction does not write to memory, and it writes a nonzero value to
> > rd. Regardless of success or failure, executing an SC.W instruction
> > *invalidates any reservation held by this hart*.
> >
> > More details, ref:
> > https://github.com/riscv/riscv-isa-manual
>
> I think section "3.5.3.2 Reservability PMA" [1] would be a more relevant
> link, as this defines memory areas that either do or do not have
> forward progress guarantees, including this part:
>
>    "When LR/SC is used for memory locations marked RsrvNonEventual,
>      software should provide alternative fall-back mechanisms used when
>      lack of progress is detected."
>
> My reading of this is that if the example you tried stalls, then either
> the PMA is not RsrvEventual, and it is wrong to rely on ll/sc on this,
> or that the PMA is marked RsrvEventual but the implementation is
> buggy.
Yes, PMA just defines physical memory region attributes, But in our
processor, when MMU is enabled (satp's value register > 2) in s-mode,
it will look at our custom PTE's attributes BIT(63) ref [1]:

   PTE format:
   | 63 | 62 | 61 | 60 | 59 | 58-8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0
     SO   C    B    SH   SE    RSW   D   A   G   U   X   W   R   V
     ^    ^    ^    ^    ^
   BIT(63): SO - Strong Order
   BIT(62): C  - Cacheable
   BIT(61): B  - Bufferable
   BIT(60): SH - Shareable
   BIT(59): SE - Security

So the memory also could be RsrvNone/RsrvEventual.

[1] https://github.com/c-sky/csky-linux/commit/e837aad23148542771794d8a2fcc52afd0fcbf88

>
> It also seems that the current "amoswap" based implementation
> would be reliable independent of RsrvEventual/RsrvNonEventual.
Yes, the hardware implementation of AMO could be different from LR/SC.
AMO could use ACE snoop holding to lock the bus in hw coherency
design, but LR/SC uses an exclusive monitor without locking the bus.

> arm64 is already in the situation of having to choose between
> two cmpxchg() implementation at runtime to allow falling back to
> a slower but more general version, but it's best to avoid that if you
> can.
Current RISC-V needn't multiple versions to select, and all AMO &
LR/SC has been defined in the spec.

RISC-V hasn't CAS instructions, and it uses LR/SC for cmpxchg. I don't
think LR/SC would be slower than CAS, and CAS is just good for code
size.

>
>          Arnd
>
> [1] http://www.five-embeddev.com/riscv-isa-manual/latest/machine.html#atomicity-pmas

--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/

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