On Thu, Jan 24, 2019 at 06:44:20PM -0800, Eric Dumazet wrote: > > > On 01/24/2019 06:34 PM, Alexei Starovoitov wrote: > > On Thu, Jan 24, 2019 at 06:29:55PM -0800, Eric Dumazet wrote: > >> > >> > >> On 01/24/2019 03:58 PM, Alexei Starovoitov wrote: > >>> On Thu, Jan 24, 2019 at 07:01:09PM +0100, Peter Zijlstra wrote: > >> > >>>> and from NMI ... > >>> > >>> progs are not preemptable and map syscall accessors have bpf_prog_active > >>> counters. > >>> So nmi/kprobe progs will not be running when syscall is running. > >>> Hence dead lock is not possible and irq_save is not needed. > >> > >> > >> Speaking of NMI, how pcpu_freelist_push() and pop() can actually work ? > >> > >> It seems bpf_get_stackid() can be called from NMI, and lockdep seems to > >> complain loudly > > > > it's a known false positive. > > https://lkml.org/lkml/2018/7/25/756 > > and the same answer as before: > > we're not going to penalize performance to shut up false positive. > > > > As far as lockdep is concerned, I do not believe we care about performance. > > How can we remove this false positive, so that lockdep stays alive even after > running bpf test_progs ?
Like do irq_save version when lockdep is on? Sure. Let's do that. That splat was bugging me for very long time. I see it every single day when I run this test before applying patches. > Let see if we understood this well. > > 1. create perf event PERF_TYPE_HARDWARE:PERF_COUNT_HW_CPU_CYCLES > 2. attach bpf probram to this event > 3. since that's a hw event, the bpf program is executed in NMI context > 4. the bpf program calls bpf_get_stackid to record the trace in a bpf map > 5. bpf_get_stackid calls pcpu_freelist_pop and pcpu_freelist_push from NMI > 6. userspace calls sys_bpf(bpf_map_lookup_elem) which calls bpf_stackmap_copy > which can call pcpu_freelist_push argh. lookup cmd is missing __this_cpu_inc(bpf_prog_active); like update/delete do. Will fix. > It seems pcpu_freelist_pop and pcpu_freelist_push are not NMI safe, > so what prevents bad things to happen ? nmi checks for bpf_prog_active==0. See bpf_overflow_handler.
