On Thu, Oct 21, 2021 at 03:05:15PM +0200, Peter Zijlstra wrote: > > There's currently a number of architectures that want/have graduated > from test-and-set locks and are looking at qspinlock. > > *HOWEVER* qspinlock is very complicated and requires a lot of an > architecture to actually work correctly. Specifically it requires > forward progress between a fair number of atomic primitives, including > an xchg16 operation, which I've seen a fair number of fundamentally > broken implementations of in the tree (specifically for qspinlock no > less). > > The benefit of qspinlock over ticket lock is also non-obvious, esp. > at low contention (the vast majority of cases in the kernel), and it > takes a fairly large number of CPUs (typically also NUMA) to make > qspinlock beat ticket locks. > > Esp. things like ARM64's WFE can move the balance a lot in favour of > simpler locks by reducing the cacheline pressure due to waiters (see > their smp_cond_load_acquire() implementation for details). > > Unless you've audited qspinlock for your architecture and found it > sound *and* can show actual benefit, simpler is better. > > Therefore provide ticket locks, which depend on a single atomic > operation (fetch_add) while still providing fairness. > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > --- > include/asm-generic/qspinlock.h | 30 +++++++++ > include/asm-generic/ticket_lock_types.h | 11 +++ > include/asm-generic/ticket_lock.h | 97 ++++++++++++++++++++++++++++++++ > 3 files changed, 138 insertions(+) Huh. I looked quite closely at this a while back but seems like I forgot to actually reply here. So, given that it doesn't seem to be in linux-next yet: Acked-by: Will Deacon <will@xxxxxxxxxx> Will
