linux-kernel - Re: [PATCH 2/2] rcu/nocb: Fix missed RCU barrier on deoffloading

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Message-ID: <871708a9-52ee-471c-b36b-35209827d8c7@amd.com>
Date: Wed, 13 Nov 2024 08:52:03 +0530
From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Zqiang <qiang.zhang1211@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Joel Fernandes <joel@...lfernandes.org>,
 Josh Triplett <josh@...htriplett.org>, Lai Jiangshan
 <jiangshanlai@...il.com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, "Paul E . McKenney" <paulmck@...nel.org>,
 Steven Rostedt <rostedt@...dmis.org>, Uladzislau Rezki <urezki@...il.com>,
 rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 2/2] rcu/nocb: Fix missed RCU barrier on deoffloading

On 11/13/2024 3:07 AM, Frederic Weisbecker wrote:
> Le Mon, Nov 11, 2024 at 01:07:16PM +0530, Neeraj Upadhyay a écrit :
>>
>>> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
>>> index 16865475120b..2605dd234a13 100644
>>> --- a/kernel/rcu/tree_nocb.h
>>> +++ b/kernel/rcu/tree_nocb.h
>>> @@ -891,7 +891,18 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>>>  	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
>>>  					    nocb_cb_wait_cond(rdp));
>>>  	if (kthread_should_park()) {
>>> -		kthread_parkme();
>>> +		/*
>>> +		 * kthread_park() must be preceded by an rcu_barrier().
>>> +		 * But yet another rcu_barrier() might have sneaked in between
>>> +		 * the barrier callback execution and the callbacks counter
>>> +		 * decrement.
>>> +		 */
>>> +		if (rdp->nocb_cb_sleep) {
>>
>> Is READ_ONCE() not required here?
> 
> No because it can't be written concurrently at this point. The value observed
> here if kthread_should_park() must have been written locally on the previous
> call to nocb_cb_wait().
> 

Ok, got it. I was not aware of any other flow (other than the one described in
this fix) which can race with it. So, asked.

- Neeraj