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

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Date:   Wed, 7 Apr 2021 18:44:34 +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 05:51:07PM +0200, Peter Zijlstra wrote:
> 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.

The longer answer is that this means you cannot share locks (or any
atomic really) across a trust boundary, which is of course exactly what
futex does.