On Tue, Jan 29, 2019 at 08:04:57PM -0800, Alexei Starovoitov wrote:
> Lockdep warns about false positive:
The report reads like:
tracepoint_probe_register()
#0 mutex_lock(&tracepoint_mutex)
tracepoint_add_func()
static_key_slow_inc()
#1 cpus_read_lock();
_cpu_up()
#1 cpus_write_lock();
...
perf_event_init_cpu()
#2 mutex_lock(&pmus_lock);
#3 mutex_lock(&ctx->mutex);
perf_ioctl()
#4 perf_event_ctx_lock();
_perf_ioctl(IOC_QUERY_BPF)
perf_event_query_prog_array()
#5 mutex_lock(&bpf_event_mutex);
bpf_probe_register()
#5 mutex_lock(&bpf_event_mutex);
__bpf_probe_register()
tracepoint_probe_register()
#0 mutex_lock(&tracepoint_mutex);
Which to me reads like an entirely valid deadlock scenario.
And note that the first and last can be combined to give:
bpf_probe_register()
#5 mutex_lock(&bpf_event_mutex);
__bpf_probe_register()
tracepoint_probe_register()
#0 mutex_lock(&tracepoint_mutex);
tracepoint_add_func()
static_key_slow_inc()
#1 cpus_read_lock();
Which generates a deadlock even without #0.
Why do you say this is not possible? All you need is 3 CPUs, one doing a
CPU online, one doing a perf ioctl() and one doing that
bpf_probe_register().
