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 ;-) thanks, jirka --- diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c index c19719f48ce0..e6126f07e85b 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_tasks_rude(); if (new) d->image_off = noff;
