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

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Message-ID: <CAADnVQ+_eBZo5yTWpEd2pdv-dd3x=KEbqU=8awbyW3=9wm9nUA@mail.gmail.com>
Date: Wed, 8 Jan 2025 19:37:18 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Waiman Long <llong@...hat.com>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, 
	Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...nel.org>, 
	Eduard Zingerman <eddyz87@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>, 
	Barret Rhoden <brho@...gle.com>, Josh Don <joshdon@...gle.com>, Dohyun Kim <dohyunkim@...gle.com>, 
	Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH bpf-next v1 12/22] rqspinlock: Add basic support for CONFIG_PARAVIRT

On Wed, Jan 8, 2025 at 6:58 PM Waiman Long <llong@...hat.com> wrote:
>
>
> On 1/8/25 9:42 PM, Alexei Starovoitov wrote:
> > On Wed, Jan 8, 2025 at 4:48 PM Waiman Long <llong@...hat.com> 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.