From: Peter Zijlstra <peterz@infradead.org>
To: paulmck@kernel.org
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
dipankar@in.ibm.com, akpm@linux-foundation.org,
mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
joel@joelfernandes.org
Subject: Re: [PATCH tip/core/rcu 1/3] rcu-tasks: *_ONCE() for rcu_tasks_cbs_head
Date: Mon, 17 Feb 2020 13:38:51 +0100 [thread overview]
Message-ID: <20200217123851.GR14914@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200215002520.15746-1-paulmck@kernel.org>
On Fri, Feb 14, 2020 at 04:25:18PM -0800, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
>
> The RCU tasks list of callbacks, rcu_tasks_cbs_head, is sampled locklessly
> by rcu_tasks_kthread() when waiting for work to do. This commit therefore
> applies READ_ONCE() to that lockless sampling and WRITE_ONCE() to the
> single potential store outside of rcu_tasks_kthread.
>
> This data race was reported by KCSAN. Not appropriate for backporting
> due to failure being unlikely.
What failure is possible here? AFAICT this is (again) one of them
load-complare-against-constant-discard patterns that are impossible to
mess up.
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> kernel/rcu/update.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 6c4b862..a27df76 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -528,7 +528,7 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
> rhp->func = func;
> raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
> needwake = !rcu_tasks_cbs_head;
> - *rcu_tasks_cbs_tail = rhp;
> + WRITE_ONCE(*rcu_tasks_cbs_tail, rhp);
> rcu_tasks_cbs_tail = &rhp->next;
> raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
> /* We can't create the thread unless interrupts are enabled. */
> @@ -658,7 +658,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
> /* If there were none, wait a bit and start over. */
> if (!list) {
> wait_event_interruptible(rcu_tasks_cbs_wq,
> - rcu_tasks_cbs_head);
> + READ_ONCE(rcu_tasks_cbs_head));
> if (!rcu_tasks_cbs_head) {
> WARN_ON(signal_pending(current));
> schedule_timeout_interruptible(HZ/10);
> --
> 2.9.5
>
next prev parent reply other threads:[~2020-02-17 12:39 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-15 0:24 [PATCH tip/core/rcu 0/3] Tasks-RCU updates for v5.7 Paul E. McKenney
2020-02-15 0:25 ` [PATCH tip/core/rcu 1/3] rcu-tasks: *_ONCE() for rcu_tasks_cbs_head paulmck
2020-02-17 12:38 ` Peter Zijlstra [this message]
2020-02-17 18:16 ` Paul E. McKenney
2020-02-18 7:56 ` Peter Zijlstra
2020-02-18 16:27 ` Paul E. McKenney
2020-02-18 20:11 ` Peter Zijlstra
2020-02-18 20:22 ` Paul E. McKenney
2020-02-18 22:45 ` Steven Rostedt
2020-02-18 22:54 ` Paul E. McKenney
2020-02-18 23:13 ` Steven Rostedt
2020-02-18 23:54 ` Paul E. McKenney
2020-02-19 0:01 ` Joel Fernandes
2020-02-19 0:16 ` Paul E. McKenney
2020-02-19 1:13 ` Joel Fernandes
2020-02-19 1:48 ` Paul E. McKenney
2020-02-17 18:23 ` Joel Fernandes
2020-02-17 18:38 ` Marco Elver
2020-02-17 19:32 ` Joel Fernandes
2020-02-15 0:25 ` [PATCH tip/core/rcu 2/3] rcu: Add missing annotation for exit_tasks_rcu_start() paulmck
2020-02-17 14:44 ` Joel Fernandes
2020-02-17 23:10 ` Paul E. McKenney
2020-02-15 0:25 ` [PATCH tip/core/rcu 3/3] rcu: Add missing annotation for exit_tasks_rcu_finish() paulmck
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