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.
