From: Uladzislau Rezki <urezki@gmail.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
rcu@vger.kernel.org, "Paul E. McKenney" <paulmck@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Mike Galbraith <efault@gmx.de>,
urezki@gmail.com
Subject: Re: [PATCH 1/3] rcu: Use static initializer for krc.lock
Date: Thu, 16 Apr 2020 17:01:43 +0200 [thread overview]
Message-ID: <20200416150038.GA7772@pc636> (raw)
In-Reply-To: <20200416144254.GC90777@google.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
next prev parent reply other threads:[~2020-04-16 15:01 UTC|newest]
Thread overview: 85+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-15 16:00 [PATCH 0/3] rcu: Static initializer + misc Sebastian Andrzej Siewior
2020-04-15 16:00 ` [PATCH 1/3] rcu: Use static initializer for krc.lock Sebastian Andrzej Siewior
2020-04-16 14:42 ` Joel Fernandes
2020-04-16 15:01 ` Uladzislau Rezki [this message]
2020-04-16 15:20 ` Sebastian Andrzej Siewior
2020-04-16 15:38 ` Uladzislau Rezki
2020-04-16 15:46 ` Sebastian Andrzej Siewior
2020-04-16 16:01 ` Uladzislau Rezki
2020-04-16 16:11 ` Sebastian Andrzej Siewior
2020-04-16 16:18 ` Uladzislau Rezki
2020-04-16 16:33 ` Sebastian Andrzej Siewior
2020-04-16 17:29 ` Paul E. McKenney
2020-04-16 18:23 ` Sebastian Andrzej Siewior
2020-04-16 18:29 ` Paul E. McKenney
2020-04-16 18:43 ` Joel Fernandes
2020-04-16 20:56 ` Sebastian Andrzej Siewior
2020-04-16 21:04 ` Joel Fernandes
2020-04-16 21:07 ` Sebastian Andrzej Siewior
2020-04-16 18:40 ` Steven Rostedt
2020-04-16 18:53 ` Joel Fernandes
2020-04-16 19:24 ` Steven Rostedt
2020-04-16 20:41 ` Joel Fernandes
2020-04-16 21:05 ` Sebastian Andrzej Siewior
2020-04-16 17:28 ` Paul E. McKenney
2020-04-16 15:18 ` Sebastian Andrzej Siewior
2020-04-16 18:41 ` Joel Fernandes
2020-04-16 18:59 ` Joel Fernandes
2020-04-16 19:26 ` Steven Rostedt
2020-04-16 19:53 ` Paul E. McKenney
2020-04-16 20:05 ` Uladzislau Rezki
2020-04-16 20:25 ` Paul E. McKenney
2020-04-16 21:02 ` Joel Fernandes
2020-04-16 21:18 ` Uladzislau Rezki
2020-04-16 21:26 ` Uladzislau Rezki
2020-04-16 21:28 ` Sebastian Andrzej Siewior
2020-04-16 20:36 ` Joel Fernandes
2020-04-16 21:00 ` Paul E. McKenney
2020-04-16 21:34 ` Sebastian Andrzej Siewior
2020-04-17 3:05 ` Joel Fernandes
2020-04-17 8:47 ` Uladzislau Rezki
2020-04-17 15:04 ` Sebastian Andrzej Siewior
2020-04-17 18:26 ` Joel Fernandes
2020-04-17 18:54 ` Paul E. McKenney
2020-04-18 12:37 ` Uladzislau Rezki
2020-04-19 14:58 ` Paul E. McKenney
2020-04-20 0:27 ` Joel Fernandes
2020-04-20 1:17 ` Joel Fernandes
2020-04-20 1:44 ` Paul E. McKenney
2020-04-20 12:13 ` Uladzislau Rezki
2020-04-20 12:36 ` joel
2020-04-20 13:00 ` Uladzislau Rezki
2020-04-20 13:26 ` Paul E. McKenney
2020-04-20 16:08 ` Uladzislau Rezki
2020-04-20 16:25 ` Paul E. McKenney
2020-04-20 16:29 ` Uladzislau Rezki
2020-04-20 16:46 ` Paul E. McKenney
2020-04-20 16:59 ` Uladzislau Rezki
2020-04-20 17:21 ` Paul E. McKenney
2020-04-20 17:40 ` Uladzislau Rezki
2020-04-20 17:57 ` Joel Fernandes
2020-04-20 18:13 ` Paul E. McKenney
2020-04-20 17:59 ` Paul E. McKenney
2020-04-20 19:06 ` Uladzislau Rezki
2020-04-20 20:17 ` Uladzislau Rezki
2020-04-20 22:16 ` Paul E. McKenney
2020-04-21 1:22 ` Steven Rostedt
2020-04-21 5:18 ` Uladzislau Rezki
2020-04-21 13:30 ` Steven Rostedt
2020-04-21 13:45 ` Uladzislau Rezki
2020-04-21 13:39 ` Sebastian Andrzej Siewior
2020-04-21 15:41 ` Paul E. McKenney
2020-04-21 17:05 ` Sebastian Andrzej Siewior
2020-04-21 18:09 ` Paul E. McKenney
2020-04-22 11:13 ` Sebastian Andrzej Siewior
2020-04-22 13:33 ` Paul E. McKenney
2020-04-22 15:46 ` Sebastian Andrzej Siewior
2020-04-22 16:19 ` Paul E. McKenney
2020-04-22 16:35 ` Paul E. McKenney
2020-04-20 3:02 ` Mike Galbraith
2020-04-20 12:30 ` joel
2020-04-17 16:11 ` Uladzislau Rezki
2020-04-19 12:15 ` Uladzislau Rezki
2020-04-15 16:00 ` [PATCH 2/3] rcu: Use consistent locking around kfree_rcu_drain_unlock() Sebastian Andrzej Siewior
2020-04-15 16:00 ` [PATCH 3/3] rcu: Avoid using xchg() in kfree_call_rcu_add_ptr_to_bulk() Sebastian Andrzej Siewior
2020-04-20 15:23 ` Joel Fernandes
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