On Sat, Dec 10, 2022 at 02:11:34PM +0100, Jiri Olsa wrote: > On Fri, Dec 09, 2022 at 05:12:03PM -0800, Alexei Starovoitov wrote: > > 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. > > synchronize_rcu seems to work as well, I'll keep the test > running for some time looks good, Hao Sun, could you please test change below? thanks, jirka --- diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c index c19719f48ce0..4b0fa5b98137 100644 --- a/kernel/bpf/dispatcher.c +++ b/kernel/bpf/dispatcher.c @@ -124,6 +124,7 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs) } __BPF_DISPATCHER_UPDATE(d, new ?: (void *)&bpf_dispatcher_nop_func); + synchronize_rcu(); if (new) d->image_off = noff;
