[PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

[Peter Zijlstra]

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.