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

Peter Zijlstra
2021-03-29T19:47:35Z

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

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.