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Date:   Mon, 29 Mar 2021 14:49:31 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Guo Ren <guoren@kernel.org>
Cc:     linux-riscv <linux-riscv@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-csky@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>,
        Guo Ren <guoren@linux.alibaba.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, Anup Patel <anup@brainfault.org>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add
 ARCH_USE_QUEUED_SPINLOCKS_XCHG32
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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.