Re: [PATCH] locking: Generic ticket lock

[Will Deacon]

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