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From: Thomas Gleixner <tglx@linutronix.de>
To: David Miller <davem@davemloft.net>
Cc: linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
	netdev@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
	bigeasy@linutronix.de, peterz@infradead.org, williams@redhat.com,
	rostedt@goodmis.org, juri.lelli@redhat.com, mingo@kernel.org
Subject: Re: [RFC patch 00/19] bpf: Make BPF and PREEMPT_RT co-exist
Date: Fri, 14 Feb 2020 19:36:37 +0100
Message-ID: <87pneht3re.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200214.095303.341559462549043464.davem@davemloft.net>

David Miller <davem@davemloft.net> writes:

> From: Thomas Gleixner <tglx@linutronix.de>
> Date: Fri, 14 Feb 2020 14:39:17 +0100
>
>> This is a follow up to the initial patch series which David posted a
>> while ago:
>> 
>>  https://lore.kernel.org/bpf/20191207.160357.828344895192682546.davem@davemloft.net/
>> 
>> which was (while non-functional on RT) a good starting point for further
>> investigations.
>
> This looks really good after a cursory review, thanks for doing this week.
>
> I was personally unaware of the pre-allocation rules for MAPs used by
> tracing et al.  And that definitely shapes how this should be handled.

Hmm. I just noticed that my analysis only holds for PERF events. But
that's broken on mainline already.

Assume the following simplified callchain:

       kmalloc() from regular non BPF context
         cache empty
           freelist empty
             lock(zone->lock);
                tracepoint or kprobe
                  BPF()
                    update_elem()
                      lock(bucket)
                        kmalloc()
                          cache empty
                            freelist empty
                              lock(zone->lock);  <- DEADLOCK

So really, preallocation _must_ be enforced for all variants of
intrusive instrumentation. There is no if and but, it's simply mandatory
as all intrusive instrumentation has to follow the only sensible
principle: KISS = Keep It Safe and Simple.

The above is a perfectly valid scenario and works with perf and tracing,
so it has to work with BPF in the same safe way.

I might be missing some magic enforcement of that, but I got lost in the
maze.

Thanks,

        tglx


  reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-14 13:39 Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 01/19] sched: Provide migrate_disable/enable() inlines Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 02/19] sched: Provide cant_migrate() Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 03/19] bpf: Update locking comment in hashtab code Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 04/19] bpf/tracing: Remove redundant preempt_disable() in __bpf_trace_run() Thomas Gleixner
2020-02-19 16:54   ` Steven Rostedt
2020-02-19 17:26     ` Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 05/19] perf/bpf: Remove preempt disable around BPF invocation Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 06/19] bpf: Dont iterate over possible CPUs with interrupts disabled Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 07/19] bpf: Provide BPF_PROG_RUN_PIN_ON_CPU() macro Thomas Gleixner
2020-02-14 18:50   ` Mathieu Desnoyers
2020-02-14 19:36     ` Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 08/19] bpf: Replace cant_sleep() with cant_migrate() Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 09/19] bpf: Use BPF_PROG_RUN_PIN_ON_CPU() at simple call sites Thomas Gleixner
2020-02-19  1:39   ` Vinicius Costa Gomes
2020-02-19  9:00     ` Thomas Gleixner
2020-02-19 16:38       ` Alexei Starovoitov
2020-02-14 13:39 ` [RFC patch 10/19] trace/bpf: Use migrate disable in trace_call_bpf() Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 11/19] bpf/tests: Use migrate disable instead of preempt disable Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 12/19] bpf: Use migrate_disable/enabe() in trampoline code Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 13/19] bpf: Use migrate_disable/enable in array macros and cgroup/lirc code Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 14/19] bpf: Use migrate_disable() in hashtab code Thomas Gleixner
2020-02-14 19:11   ` Mathieu Desnoyers
2020-02-14 19:56     ` Thomas Gleixner
2020-02-18 23:36       ` Alexei Starovoitov
2020-02-19  0:49         ` Thomas Gleixner
2020-02-19  1:23           ` Alexei Starovoitov
2020-02-19 15:17         ` Mathieu Desnoyers
2020-02-20  4:19           ` Alexei Starovoitov
2020-02-14 13:39 ` [RFC patch 15/19] bpf: Use migrate_disable() in sys_bpf() Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 16/19] bpf: Factor out hashtab bucket lock operations Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 17/19] bpf: Prepare hashtab locking for PREEMPT_RT Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 18/19] bpf, lpm: Make locking RT friendly Thomas Gleixner
2020-02-14 13:39 ` [RFC patch 19/19] bpf/stackmap: Dont trylock mmap_sem with PREEMPT_RT and interrupts disabled Thomas Gleixner
2020-02-14 17:53 ` [RFC patch 00/19] bpf: Make BPF and PREEMPT_RT co-exist David Miller
2020-02-14 18:36   ` Thomas Gleixner [this message]
2020-02-17 12:59     ` [PATCH] bpf: Enforce map preallocation for all instrumentation programs Thomas Gleixner
2020-02-15 20:09 ` [RFC patch 00/19] bpf: Make BPF and PREEMPT_RT co-exist Jakub Kicinski

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