linux-kernel - Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS

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Message-ID: <YGHM2/s4FpWZiEQ6@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Mar 2021 14:49:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Anup Patel <anup@...infault.org>
Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add
 ARCH_USE_QUEUED_SPINLOCKS_XCHG32

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.