From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: rostedt <rostedt@goodmis.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
"Joel Fernandes, Google" <joel@joelfernandes.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
Thomas Gleixner <tglx@linutronix.de>,
paulmck <paulmck@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook
Date: Tue, 11 Feb 2020 16:46:45 +0100 [thread overview]
Message-ID: <20200211154645.GX14914@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <504801580.617591.1581435278202.JavaMail.zimbra@efficios.com>
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.
next prev parent reply other threads:[~2020-02-11 15:47 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-11 14:50 [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook Steven Rostedt
2020-02-11 15:34 ` Mathieu Desnoyers
2020-02-11 15:46 ` Peter Zijlstra [this message]
2020-02-11 16:02 ` Mathieu Desnoyers
2020-02-11 15:34 ` Peter Zijlstra
2020-02-11 16:18 ` Steven Rostedt
2020-02-11 16:27 ` Mathieu Desnoyers
2020-02-11 16:35 ` Steven Rostedt
2020-02-11 17:29 ` Peter Zijlstra
2020-02-11 17:32 ` Peter Zijlstra
2020-02-11 18:54 ` Paul E. McKenney
2020-02-12 8:05 ` Peter Zijlstra
2020-02-12 9:05 ` Paul E. McKenney
2020-02-11 17:35 ` Mathieu Desnoyers
2020-02-12 8:02 ` Peter Zijlstra
2020-02-12 15:14 ` Mathieu Desnoyers
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