From: Joel Fernandes <joel@joelfernandes.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
Christian Brauner <christian@brauner.io>,
Daniel Borkmann <daniel@iogearbox.net>,
David Ahern <dsahern@gmail.com>,
"David S. Miller" <davem@davemloft.net>,
Ido Schimmel <idosch@mellanox.com>,
Ingo Molnar <mingo@redhat.com>,
"moderated list:INTEL ETHERNET DRIVERS"
<intel-wired-lan@lists.osuosl.org>,
Jakub Kicinski <jakub.kicinski@netronome.com>,
Jeff Kirsher <jeffrey.t.kirsher@intel.com>,
Jesper Dangaard Brouer <hawk@kernel.org>,
John Fastabend <john.fastabend@gmail.com>,
Josh Triplett <josh@joshtriplett.org>,
keescook@chromium.org, Lai Jiangshan <jiangshanlai@gmail.com>,
Martin KaFai Lau <kafai@fb.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
netdev@vger.kernel.org,
"Paul E. McKenney" <paulmck@linux.ibm.com>,
rcu@vger.kernel.org, Song Liu <songliubraving@fb.com>,
Steven Rostedt <rostedt@goodmis.org>,
xdp-newbies@vger.kernel.org, Yonghong Song <yhs@fb.com>
Subject: Re: [PATCH RFC 3/5] sched/cpufreq: Fix incorrect RCU API usage
Date: Thu, 21 Feb 2019 10:21:39 -0500 [thread overview]
Message-ID: <20190221152139.GB19213@google.com> (raw)
In-Reply-To: <20190221091805.GX32477@hirez.programming.kicks-ass.net>
On Thu, Feb 21, 2019 at 10:18:05AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 21, 2019 at 12:49:40AM -0500, Joel Fernandes (Google) wrote:
> > @@ -34,8 +34,12 @@ void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
> > if (WARN_ON(!data || !func))
> > return;
> >
> > - if (WARN_ON(per_cpu(cpufreq_update_util_data, cpu)))
> > + rcu_read_lock();
> > + if (WARN_ON(rcu_dereference(per_cpu(cpufreq_update_util_data, cpu)))) {
> > + rcu_read_unlock();
> > return;
> > + }
> > + rcu_read_unlock();
> >
> > data->func = func;
> > rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
>
> This doesn't make any kind of sense to me.
>
As per the rcu_assign_pointer() line, I inferred that
cpufreq_update_util_data is expected to be RCU protected. Reading the pointer
value of RCU pointers generally needs to be done from RCU read section, and
using rcu_dereference() (or using rcu_access()).
In this patch, I changed cpufreq_update_util_data to be __rcu annotated to
avoid the sparse error thrown by rcu_assign_pointer().
Instead of doing that, If your intention here is RELEASE barrier, should I
just replace in this function:
rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
with:
smp_store_release(per_cpu(cpufreq_update_util_data, cpu), data))
?
It would be nice IMO to be explicit about the intention of release/publish
semantics by using smp_store_release().
thanks,
- Joel
next prev parent reply other threads:[~2019-02-21 15:21 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-21 5:49 [PATCH RFC 0/5] RCU fixes for rcu_assign_pointer() usage Joel Fernandes (Google)
2019-02-21 5:49 ` [PATCH RFC 1/5] net: rtnetlink: Fix incorrect RCU API usage Joel Fernandes (Google)
2019-02-21 5:49 ` [PATCH RFC 2/5] ixgbe: " Joel Fernandes (Google)
2019-02-21 5:49 ` [PATCH RFC 3/5] sched/cpufreq: " Joel Fernandes (Google)
2019-02-21 9:18 ` Peter Zijlstra
2019-02-21 15:21 ` Joel Fernandes [this message]
2019-02-21 15:31 ` Peter Zijlstra
2019-02-21 15:52 ` Paul E. McKenney
2019-02-21 16:11 ` Peter Zijlstra
2019-02-21 17:13 ` Joel Fernandes
2019-02-21 17:29 ` Paul E. McKenney
2019-02-21 16:17 ` Steven Rostedt
2019-02-21 23:05 ` Rafael J. Wysocki
2019-02-21 5:49 ` [PATCH RFC 4/5] sched/topology: Annonate RCU pointers properly Joel Fernandes (Google)
2019-02-21 9:19 ` Peter Zijlstra
2019-02-21 15:10 ` Joel Fernandes
2019-02-21 15:29 ` Peter Zijlstra
2019-02-21 17:17 ` Joel Fernandes
2019-02-21 5:49 ` [PATCH RFC 5/5] rcuwait: Replace rcu_assign_pointer() with WRITE_ONCE Joel Fernandes (Google)
2019-02-21 9:20 ` Peter Zijlstra
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