linux-kernel - Re: [RFC patch 04/19] bpf/tracing: Remove redundant preempt_disable() in __bpf_trace

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Message-ID: <20200219115415.57ee6d3c@gandalf.local.home>
Date:   Wed, 19 Feb 2020 11:54:15 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Sebastian Sewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Clark Williams <williams@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC patch 04/19] bpf/tracing: Remove redundant
 preempt_disable() in __bpf_trace_run()

On Fri, 14 Feb 2020 14:39:21 +0100
Thomas Gleixner <tglx@...utronix.de> wrote:

> __bpf_trace_run() disables preemption around the BPF_PROG_RUN() invocation.
> 
> This is redundant because __bpf_trace_run() is invoked from a trace point
> via __DO_TRACE() which already disables preemption _before_ invoking any of
> the functions which are attached to a trace point.
> 
> Remove it.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/trace/bpf_trace.c |    2 --
>  1 file changed, 2 deletions(-)
> 
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1476,9 +1476,7 @@ static __always_inline
>  void __bpf_trace_run(struct bpf_prog *prog, u64 *args)
>  {

Should there be a "cant_migrate()" added here?

-- Steve

>  	rcu_read_lock();
> -	preempt_disable();
>  	(void) BPF_PROG_RUN(prog, args);
> -	preempt_enable();
>  	rcu_read_unlock();
>  }
>