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

[Waiman Long]

On 1/8/25 10:37 PM, Alexei Starovoitov wrote:
> On Wed, Jan 8, 2025 at 6:58 PM Waiman Long <llong@xxxxxxxxxx> wrote:
> > 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.
> Agreed.
>
> > > 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.
> Let's not open this can of worms.
> There will be a proper watchdog eventually.
> If we start to warn, when do we warn? On any bpf program loaded?
> How about classic BPF ? tcpdump and seccomp ? They are limited
> to 4k instructions, but folks can abuse that too.

My intention is to document this somewhere, not to print out a warning 
in the kernel dmesg log.

Cheers,
Longman