linux-kernel - Re: [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT

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Date:   Sun, 22 Aug 2021 19:15:32 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        rcu@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Steven Price <steven.price@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT

On 17/08/21 17:36, Sebastian Andrzej Siewior wrote:
>> +static inline int rcu_lockdep_is_held_nocb_local(struct rcu_data *rdp)
>> +{
>> +	return lockdep_is_held(
>> +#ifdef CONFIG_PREEMPT_RT
>> +		&rdp->nocb_local_lock.lock
>> +#else
>> +		&rdp->nocb_local_lock
>> +#endif
>> +	);
>> +}
>
> Now that I see it and Paul asked for it, please just use !RT version.
>       return lockdep_is_held(&rdp->nocb_local_lock);
>
> and RT will work, too.
>

The above was required due to the (previous) definition of local_lock_t:

  typedef struct {
          spinlock_t		lock;
  } local_lock_t;

On -rt11 I see this is now:

  typedef spinlock_t local_lock_t;

which indeed means the iffdefery can (actually it *has* to) go.

>>  static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
>>  {
>>      /* Race on early boot between thread creation and assignment */
>> @@ -1629,6 +1664,22 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
>>      }
>>  }
>>
>> +/*
>> + * The invocation of rcu_core() within the RCU core kthreads remains preemptible
>> + * under PREEMPT_RT, thus the offload state of a CPU could change while
>> + * said kthreads are preempted. Prevent this from happening by protecting the
>> + * offload state with a local_lock().
>> + */
>> +static void rcu_nocb_local_lock(struct rcu_data *rdp)
>> +{
>> +	local_lock(&rcu_data.nocb_local_lock);
>> +}
>> +
>> +static void rcu_nocb_local_unlock(struct rcu_data *rdp)
>> +{
>> +	local_unlock(&rcu_data.nocb_local_lock);
>> +}
>> +
> Do you need to pass rdp given that it is not used?
>

Not anymore, you're right.

>>  /* Lockdep check that ->cblist may be safely accessed. */
>>  static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
>>  {
>
> Sebastian