On Thu, Feb 23, 2023 at 08:43:05AM +0000, Zhang, Qiang1 wrote:
> > From: Zqiang <qiang1.zhang@xxxxxxxxx>
> > Sent: Thursday, February 23, 2023 2:30 PM
> > To: paulmck@xxxxxxxxxx; frederic@xxxxxxxxxx; quic_neeraju@xxxxxxxxxxx;
> > joel@xxxxxxxxxxxxxxxxx
> > Cc: rcu@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Subject: [PATCH] rcu-tasks: Directly invoke rcuwait_wake_up() in
> > call_rcu_tasks_generic()
> >
> > According to commit '3063b33a347c ("Avoid raw-spinlocked wakeups from
> > call_rcu_tasks_generic()")', the grace-period kthread is delayed to wakeup
> > using irq_work_queue() is because if the caller of
> > call_rcu_tasks_generic() holds a raw spinlock, when the kernel is built with
> > CONFIG_PROVE_RAW_LOCK_NESTING=y, due to a spinlock will be hold in
> > wake_up(), so the lockdep splats will happen. but now using
> > rcuwait_wake_up() to wakeup grace-period kthread instead of wake_up(), in
> > rcuwait_wake_up() no spinlock will be acquired, so this commit remove using
> >
> >There are still spinlock-acquisition and spinlock-release invocations within the call path from rcuwait_wake_up().
> >
> >rcuwait_wake_up() -> wake_up_process() -> try_to_wake_up(), then:
> >
> > raw_spin_lock_irqsave()
> > ...
> > raw_spin_unlock_irqrestore
>
> Yes, but this is raw_spinlock acquisition and release(note: spinlock will convert to
> sleepable lock in Preempt-RT kernel, but raw spinlock is not change).
>
> acquire raw_spinlock -> acquire spinlock will trigger lockdep warning.
Is this really safe in the long run though? I seem to remember there are
weird locking dependencies if RCU is used from within the scheduler [1].
I prefer to keep it as irq_work_queue() unless you are seeing some benefit.
Generally, there has to be a 'win' or other justification for adding more
risk.
thanks,
- Joel
[1] http://www.joelfernandes.org/rcu/scheduler/locking/2019/09/02/rcu-schedlocks.html
> > irq_work_queue(), invoke rcuwait_wake_up() directly in
> > call_rcu_tasks_generic().
> >
> > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> > ---
> > kernel/rcu/tasks.h | 16 +---------------
> > 1 file changed, 1 insertion(+), 15 deletions(-)
> >
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index
> > baf7ec178155..757b8c6da1ad 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -39,7 +39,6 @@ struct rcu_tasks_percpu {
> > unsigned long rtp_jiffies;
> > unsigned long rtp_n_lock_retries;
> > struct work_struct rtp_work;
> > - struct irq_work rtp_irq_work;
> > struct rcu_head barrier_q_head;
> > struct list_head rtp_blkd_tasks;
> > int cpu;
> > @@ -112,12 +111,9 @@ struct rcu_tasks {
> > char *kname;
> > };
> >
> > -static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);
> > -
> > #define DEFINE_RCU_TASKS(rt_name, gp, call, n)
> > \
> > static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = {
> > \
> > .lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ##
> > __percpu.cbs_pcpu_lock), \
> > - .rtp_irq_work = IRQ_WORK_INIT_HARD(call_rcu_tasks_iw_wakeup),
> > \
> > };
> > \
> > static struct rcu_tasks rt_name =
> > \
> > {
> > \
> > @@ -273,16 +269,6 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
> > pr_info("%s: Setting shift to %d and lim to %d.\n", __func__,
> > data_race(rtp->percpu_enqueue_shift), data_race(rtp-
> > >percpu_enqueue_lim));
> > }
> >
> > -// IRQ-work handler that does deferred wakeup for call_rcu_tasks_generic().
> > -static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp) -{
> > - struct rcu_tasks *rtp;
> > - struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct
> > rcu_tasks_percpu, rtp_irq_work);
> > -
> > - rtp = rtpcp->rtpp;
> > - rcuwait_wake_up(&rtp->cbs_wait);
> > -}
> > -
> > // Enqueue a callback for the specified flavor of Tasks RCU.
> > static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
> > struct rcu_tasks *rtp)
> > @@ -334,7 +320,7 @@ static void call_rcu_tasks_generic(struct rcu_head
> > *rhp, rcu_callback_t func,
> > rcu_read_unlock();
> > /* We can't create the thread unless interrupts are enabled. */
> > if (needwake && READ_ONCE(rtp->kthread_ptr))
> > - irq_work_queue(&rtpcp->rtp_irq_work);
> > + rcuwait_wake_up(&rtp->cbs_wait);
> > }
> >
> > // RCU callback function for rcu_barrier_tasks_generic().
> > --
> > 2.25.1
>
