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

On Mon, Mar 29, 2021 at 12:13:10PM +0000, Anup Patel wrote:

We had discussions in the RISC-V platforms group about this. Over there,
We had evaluated all spin lock approaches (ticket, qspinlock, etc) tried
in Linux till now. It was concluded in those discussions that eventually we
have to move to qspinlock (even if we moved to ticket spinlock temporarily)
because qspinlock avoids cache line bouncing. Also, moving to qspinlock
will be aligned with other major architectures supported in Linux (such as
x86, ARM64)

Some of the organizations working on high-end RISC-V systems (> 32
CPUs) are interested in having an optimized spinlock implementation
(just like other major architectures x86 and ARM64).

Based on above, Linux RISC-V should move to qspinlock.

That's all well and good, but first you need architectural forward
progress guarantees. Otherwise there's absolutely no point in building
complex locking primitives.

And unless you already have such big systems in silicon, where you
can benchmark against simpler locks (like ticket) there really is no
point in the complexity.