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

[Alexei Starovoitov]

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.

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.