On Fri, Dec 9, 2022 at 4:06 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote: > > On Fri, Dec 09, 2022 at 03:34:45PM -0800, Jakub Kicinski wrote: > > On Sat, 10 Dec 2022 00:32:07 +0100 Daniel Borkmann wrote: > > > fwiw, these should not be necessary, Documentation/RCU/checklist.rst : > > > > > > [...] One example of non-obvious pairing is the XDP feature in networking, > > > which calls BPF programs from network-driver NAPI (softirq) context. BPF > > > relies heavily on RCU protection for its data structures, but because the > > > BPF program invocation happens entirely within a single local_bh_disable() > > > section in a NAPI poll cycle, this usage is safe. The reason that this usage > > > is safe is that readers can use anything that disables BH when updaters use > > > call_rcu() or synchronize_rcu(). [...] > > > > FWIW I sent a link to the thread to Paul and he confirmed > > the RCU will wait for just the BH. > > so IIUC we can omit the rcu_read_lock/unlock on bpf_prog_run_xdp side > > Paul, > any thoughts on what we can use in here to synchronize bpf_dispatcher_change_prog > with bpf_prog_run_xdp callers? > > with synchronize_rcu_tasks I'm getting splats like: > https://lore.kernel.org/bpf/20221209153445.22182ca5@xxxxxxxxxx/T/#m0a869f93404a2744884d922bc96d497ffe8f579f > > synchronize_rcu_tasks_rude seems to work (patch below), but it also sounds special ;-) Jiri, I haven't tried to repro this yet, but I feel you're on the wrong path here. The splat has this: ? bpf_prog_run_xdp include/linux/filter.h:775 [inline] ? bpf_test_run+0x2ce/0x990 net/bpf/test_run.c:400 that test_run logic takes rcu_read_lock. See bpf_test_timer_enter. I suspect the addition of synchronize_rcu_tasks_rude only slows down the race. The synchronize_rcu_tasks_trace also behaves like synchronize_rcu. See our new and fancy rcu_trace_implies_rcu_gp(), but I'm not sure it applies to synchronize_rcu_tasks_rude. Have you tried with just synchronize_rcu() ? If your theory about the race is correct then the vanila sync_rcu should help. If not, the issue is some place else.
