-----Original Message-----
From: Guo Ren <guoren@kernel.org>
Sent: 30 March 2021 08:44
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-riscv <redacted>; Linux Kernel Mailing List
[off-list ref]; linux-csky@vger.kernel.org; linux-arch
[off-list ref]; Guo Ren [off-list ref]; Will
Deacon [off-list ref]; Ingo Molnar [off-list ref]; Waiman
Long [off-list ref]; Arnd Bergmann [off-list ref]; Anup
Patel [off-list ref]
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add
ARCH_USE_QUEUED_SPINLOCKS_XCHG32

On Mon, Mar 29, 2021 at 8:50 PM Peter Zijlstra [off-list ref]
wrote:

quoted

On Mon, Mar 29, 2021 at 08:01:41PM +0800, Guo Ren wrote:

quoted

u32 a = 0x55aa66bb;
u16 *ptr = &a;

CPU0                       CPU1
=========             =========
xchg16(ptr, new)     while(1)
                                    WRITE_ONCE(*(ptr + 1), x);

When we use lr.w/sc.w implement xchg16, it'll cause CPU0 deadlock.

Then I think your LL/SC is broken.

That also means you really don't want to build super complex locking
primitives on top, because that live-lock will percolate through.

Do you mean the below implementation has live-lock risk?
+static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
+{
+       u32 old, new, val = atomic_read(&lock->val);
+
+       for (;;) {
+               new = (val & _Q_LOCKED_PENDING_MASK) | tail;
+               old = atomic_cmpxchg(&lock->val, val, new);
+               if (old == val)
+                       break;
+
+               val = old;
+       }
+       return old;
+}

quoted

Step 1 would be to get your architecute fixed such that it can provide
fwd progress guarantees for LL/SC. Otherwise there's absolutely no
point in building complex systems with it.

Quote Waiman's comment [1] on xchg16 optimization:

"This optimization is needed to make the qspinlock achieve performance
parity with ticket spinlock at light load."

[1] https://lore.kernel.org/kvm/1429901803-29771-6-git-send-email-
Waiman.Long@hp.com/

So for a non-xhg16 machine:
 - ticket-lock for small numbers of CPUs
 - qspinlock for large numbers of CPUs

Okay, I'll put all of them into the next patch 

I would suggest to have separate Kconfig opitons for ticket spinlock
in Linux RISC-V which will be disabled by default. This means Linux
RISC-V will use qspinlock by default and use ticket spinlock only when
ticket spinlock kconfig is enabled.

Regards,
Anup