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

Arnd Bergmann
2021-03-29T22:42:09Z

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.

It also seems that the current "amoswap" based implementation
would be reliable independent of RsrvEventual/RsrvNonEventual.
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.

Arnd

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