netdev - Re: [PATCH bpf-next 3/4] bpf: fix lockdep false positive in bpf_prog

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Date:   Wed, 30 Jan 2019 11:10:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <ast@...nel.org>
Cc:     davem@...emloft.net, daniel@...earbox.net, edumazet@...gle.com,
        jannh@...gle.com, netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH bpf-next 3/4] bpf: fix lockdep false positive in
 bpf_prog_register

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().