Re: [PATCH 1/3] rcu: Use static initializer for krc.lock

[Uladzislau Rezki <urezki@gmail.com>]

Hello.

I am not fully in the picture(next time will be :)). But see some comments below.

> > The per-CPU variable is initialized at runtime in
> > kfree_rcu_batch_init(). This function is invoked before
> > `rcu_scheduler_active' is set to `RCU_SCHEDULER_RUNNING'. After the
> > initialisation, `->initialized' is to true.
> > 
> > The spin_lock is only acquired if `->initialized' is set to true. The
> > worqueue item is only used if `rcu_scheduler_active' set to
> > RCU_SCHEDULER_RUNNING' which happens after initialisation.
> > 
> > Use a static initializer for krc.lock and remove the runtime
> > initialisation of the lock. Since the lock can now be always acquired,
> > remove the `->initialized' check.
> > The local_irq_save() is removed and raw_cpu_ptr() + spin_lock_irqsave()
> > is used. The worst case scenario is that after raw_cpu_ptr() the code
> > has been moved to another CPU. This is "okay" because the data strucure
> > itself is protected with a lock.
> > Add a warning in kfree_rcu_batch_init() to ensure that this function is
> > invoked before `RCU_SCHEDULER_RUNNING' to ensure that the worker is not
> > used earlier.
> > 
> > Reported-by: Mike Galbraith <efault@gmx.de>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> >  kernel/rcu/tree.c | 22 +++++++---------------
> >  1 file changed, 7 insertions(+), 15 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index f288477ee1c26..5b0b63dd04b02 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2893,7 +2893,6 @@ struct kfree_rcu_cpu_work {
> >   * @lock: Synchronize access to this structure
> >   * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
> >   * @monitor_todo: Tracks whether a @monitor_work delayed work is pending
> > - * @initialized: The @lock and @rcu_work fields have been initialized
> >   *
> >   * This is a per-CPU structure.  The reason that it is not included in
> >   * the rcu_data structure is to permit this code to be extracted from
> > @@ -2908,12 +2907,13 @@ struct kfree_rcu_cpu {
> >  	spinlock_t lock;
> >  	struct delayed_work monitor_work;
> >  	bool monitor_todo;
> > -	bool initialized;
> >  	// Number of objects for which GP not started
> >  	int count;
> >  };
> >  
> > -static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc);
> > +static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
> > +	.lock	= __SPIN_LOCK_UNLOCKED(krc.lock),
> > +};
> >  
> >  static __always_inline void
> >  debug_rcu_head_unqueue_bulk(struct rcu_head *head)
> > @@ -3080,9 +3080,6 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
> >  {
> >  	struct kfree_rcu_bulk_data *bnode;
> >  
> > -	if (unlikely(!krcp->initialized))
> > -		return false;
> > -
> >  	lockdep_assert_held(&krcp->lock);
It makes sense to initialize the spinlock statically, i mean not in runtime.

> >  
> >  	/* Check if a new block is required. */
> > @@ -3139,10 +3136,8 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> >  	unsigned long flags;
> >  	struct kfree_rcu_cpu *krcp;
> >  
> > -	local_irq_save(flags);	// For safely calling this_cpu_ptr().
> > -	krcp = this_cpu_ptr(&krc);
> > -	if (krcp->initialized)
> > -		spin_lock(&krcp->lock);
> > +	krcp = raw_cpu_ptr(&krc);
> > +	spin_lock_irqsave(&krcp->lock, flags);
> 
It is not a good way to access to per-CPU variable. There is a race in
your code. So, we will rework it anyway soon. To guarantee that we stay
on the same CPU, first we disable IRQ's then we access per-CPU var and
take a spinlock.

--
Vlad Rezki