linux-kernel - Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS

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Date:   Tue, 30 Mar 2021 11:21:12 +0530
From:   Anup Patel <anup@...infault.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Tue, Mar 30, 2021 at 7:56 AM Guo Ren <guoren@...nel.org> wrote:
>
> On Mon, Mar 29, 2021 at 9:56 PM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Mon, Mar 29, 2021 at 2:52 PM Guo Ren <guoren@...nel.org> wrote:
> > >
> > > On Mon, Mar 29, 2021 at 7:31 PM Peter Zijlstra <peterz@...radead.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

Is this about your C-sky architecture or your RISC-V implementation.

If these PTE bits are in your RISC-V implementation then clearly your
RISC-V implementation is not compliant with the RISC-V privilege spec
because these bits are not defined in RISC-V privilege spec.

Regards,
Anup
>
> >
> > 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/