Re: [PATCH bpf-next v1 12/22] rqspinlock: Add basic support for CONFIG_PARAVIRT

[Waiman Long]

On 1/8/25 9:42 PM, Alexei Starovoitov wrote:
> On Wed, Jan 8, 2025 at 4:48 PM Waiman Long <llong@xxxxxxxxxx> wrote:
> > Is the intention to only replace raw_spinlock_t by rqspinlock but never
> > spinlock_t?
> Correct. We brainstormed whether we can introduce resilient mutex
> for sleepable context, but it's way out of scope and PI
> considerations are too complex to think through.
> rqspinlock is a spinning lock, so it's a replacement for raw_spin_lock
> and really only for bpf use cases.
Thank for the confirmation. I think we should document the fact that 
rqspinlock is a replacement for raw_spin_lock only in the rqspinlock.c 
file to prevent possible abuse in the future.
> We considered placing rqspinlock.c in kernel/bpf/ directory
> to discourage any other use beyond bpf,
> but decided to keep in kernel/locking/ only because
> it's using mcs_spinlock.h and qspinlock_stat.h
> and doing #include "../locking/mcs_spinlock.h"
> is kinda ugly.
>
> Patch 16 does:
> +++ b/kernel/locking/Makefile
> @@ -24,6 +24,9 @@  obj-$(CONFIG_SMP) += spinlock.o
>   obj-$(CONFIG_LOCK_SPIN_ON_OWNER) += osq_lock.o
>   obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
>   obj-$(CONFIG_QUEUED_SPINLOCKS) += qspinlock.o
> +ifeq ($(CONFIG_BPF_SYSCALL),y)
> +obj-$(CONFIG_QUEUED_SPINLOCKS) += rqspinlock.o
> +endif
>
> so that should give enough of a hint that it's for bpf usage.
>
> > As for the locking semantics allowed by the BPF verifier, is it possible
> > to enforce the strict locking rules for PREEMPT_RT kernel and use the
> > relaxed semantics for non-PREEMPT_RT kernel. We don't want the loading
> > of an arbitrary BPF program to break the latency guarantee of a
> > PREEMPT_RT kernel.
> Not really.
> root can load silly bpf progs that take significant
> amount time without abusing spinlocks.
> Like 100k integer divides or a sequence of thousands of calls to map_update.
> Long runtime of broken progs is a known issue.
> We're working on a runtime termination check/watchdog that
> will detect long running progs and will terminate them.
> Safe termination is tricky, as you can imagine.

Right.

In that case, we just have to warn users that they can load BPF prog at 
their own risk and PREEMPT_RT kernel may break its latency guarantee.

Thanks,
Longman