On Sat, Dec 10, 2022 at 01:06:16AM +0100, Jiri Olsa 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 ;-) It sounds like we are all talking past each other, leaving me no choice but to supply a wall of text: It is quite true that synchronize_rcu_tasks_rude() will wait for bh-disabled regions of code, just like synchronize_rcu() and synchronize_rcu_tasks() will. However, please note that synchronize_rcu_tasks() never waits on any of the idle tasks. So the usual approach in tracing is to do both a synchronize_rcu_tasks() and synchronize_rcu_tasks_rude(). One way of overlapping the resulting pair of grace periods is to use synchronize_rcu_mult(). But none of these permit readers to sleep. That is what synchronize_rcu_tasks_trace() is for, but unlike both synchronize_rcu_tasks() and synchronize_rcu_tasks_rude(), you must explicitly mark the readers with rcu_read_lock_trace() and rcu_read_unlock_trace(). This is used to protect sleepable BPF programs. Now, synchronize_rcu() will also wait on bh-disabled lines of code, with the exception of such code in the exception path, way deep in the idle loop, early in the CPU-online process, or late in the CPU-offline process. You can recognize the first two categories of code by the noinstr tags on the functions. And yes, synchronize_rcu_rude() is quite special. ;-) Does this help, or am I simply adding to the confusion? Thanx, Paul > 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;
