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Date:   Wed, 7 Apr 2021 18:44:34 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Christoph =?iso-8859-1?Q?M=FCllner?= <christophm30@gmail.com>
Cc:     Christoph Hellwig <hch@infradead.org>, Guo Ren <guoren@kernel.org>,
        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 Wed, Apr 07, 2021 at 05:51:07PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 07, 2021 at 04:29:12PM +0200, Christoph M?llner 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.