linux-kernel - Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit

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Date:   Tue, 11 Feb 2020 16:46:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to
 perf trace point hook

On Tue, Feb 11, 2020 at 10:34:38AM -0500, Mathieu Desnoyers wrote:
> 
> I'm puzzled by this function. It does:
> 
> perf_tp_event(...)
> {
>      hlist_for_each_entry_rcu(event, head, hlist_entry) {
>          ...
>      }
>      if (task && task != current) {
>          rcu_read_lock();
>          ... = rcu_dereference();
>          list_for_each_entry_rcu(...) {
>              ....
>          }
>          rcu_read_unlock();
>      }
> }
> 
> What is the purpose of the rcu_read_lock/unlock within the if (),
> considering that there is already an hlist rcu iteration just before ?
> It seems to assume that a RCU read-side of some kind of already
> ongoing.

IIRC the hlist_for_each_entry_rcu() uses the RCU stuff from the
tracepoint API, while the stuff inside the if() uses regular RCU.

Them were note the same one -- tracepoints used rcu-sched, perf used
rcu.