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

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Message-ID: <YG3U677P9QKqFGMY@hirez.programming.kicks-ass.net>
Date:   Wed, 7 Apr 2021 17:51:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christoph Müllner <christophm30@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>, Guo Ren <guoren@...nel.org>,
        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 Wed, Apr 07, 2021 at 04:29:12PM +0200, Christoph Müllner wrote:
> Further, it is not the case that RISC-V has no guarantees at all.
> It just does not provide a forward progress guarantee for a
> synchronization implementation,
> that writes in an endless loop to a memory location while trying to
> complete an LL/SC
> loop on the same memory location at the same time.

Userspace can DoS the kernel that way, see futex.