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[73.241.150.70]) by smtp.gmail.com with ESMTPSA id p7sm33285157pfa.22.2019.01.24.18.44.21 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 24 Jan 2019 18:44:21 -0800 (PST) Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock To: Alexei Starovoitov Cc: Peter Zijlstra , Alexei Starovoitov , davem@davemloft.net, daniel@iogearbox.net, jakub.kicinski@netronome.com, netdev@vger.kernel.org, kernel-team@fb.com, mingo@redhat.com, will.deacon@arm.com, Paul McKenney , jannh@google.com References: <20190124041403.2100609-1-ast@kernel.org> <20190124041403.2100609-2-ast@kernel.org> <20190124180109.GA27771@hirez.programming.kicks-ass.net> <20190124235857.xyb5xx2ufr6x5mbt@ast-mbp.dhcp.thefacebook.com> <395a3741-70c9-c345-08a4-77bc3bd3cae2@gmail.com> <20190125023402.34a5k62furpdismi@ast-mbp.dhcp.thefacebook.com> From: Eric Dumazet Message-ID: <9e1fa851-e189-ab17-ae34-236cc6b5a8b4@gmail.com> Date: Thu, 24 Jan 2019 18:44:20 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20190125023402.34a5k62furpdismi@ast-mbp.dhcp.thefacebook.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 ? 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 It seems pcpu_freelist_pop and pcpu_freelist_push are not NMI safe, so what prevents bad things to happen ? Thanks.