From: Uladzislau Rezki <urezki@gmail.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
"Paul E . McKenney" <paulmck@kernel.org>,
RCU <rcu@vger.kernel.org>,
Neeraj upadhyay <Neeraj.Upadhyay@amd.com>,
Boqun Feng <boqun.feng@gmail.com>,
Hillf Danton <hdanton@sina.com>,
Joel Fernandes <joel@joelfernandes.org>,
LKML <linux-kernel@vger.kernel.org>,
Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Subject: Re: [PATCH v5 2/4] rcu: Reduce synchronize_rcu() latency
Date: Wed, 28 Feb 2024 19:04:21 +0100 [thread overview]
Message-ID: <Zd91pR0fjiCUZTtP@pc636> (raw)
In-Reply-To: <Zd0ZtNu+Rt0qXkfS@lothringen>
On Tue, Feb 27, 2024 at 12:07:32AM +0100, Frederic Weisbecker wrote:
> On Tue, Feb 20, 2024 at 07:31:13PM +0100, Uladzislau Rezki (Sony) wrote:
> > +static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> > +{
> > + struct llist_node *done, *rcu, *next, *head;
> > +
> > + /*
> > + * This work execution can potentially execute
> > + * while a new done tail is being updated by
> > + * grace period kthread in rcu_sr_normal_gp_cleanup().
> > + * So, read and updates of done tail need to
> > + * follow acq-rel semantics.
> > + *
> > + * Given that wq semantics guarantees that a single work
> > + * cannot execute concurrently by multiple kworkers,
> > + * the done tail list manipulations are protected here.
> > + */
> > + done = smp_load_acquire(&rcu_state.srs_done_tail);
> > + if (!done)
> > + return;
> > +
> > + WARN_ON_ONCE(!rcu_sr_is_wait_head(done));
> > + head = done->next;
> > + done->next = NULL;
>
> Can the following race happen?
>
> CPU 0 CPU 1
> ----- -----
>
> // wait_tail == HEAD1
> rcu_sr_normal_gp_cleanup() {
> // has passed SR_MAX_USERS_WAKE_FROM_GP
> wait_tail->next = next;
> // done_tail = HEAD1
> smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> queue_work() {
> test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
> __queue_work()
> }
> }
>
> set_work_pool_and_clear_pending()
> rcu_sr_normal_gp_cleanup_work() {
> // new GP, wait_tail == HEAD2
> rcu_sr_normal_gp_cleanup() {
> // executes all completion, but stop at HEAD1
> wait_tail->next = HEAD1;
> // done_tail = HEAD2
> smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> queue_work() {
> test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)
> __queue_work()
> }
> }
> // done = HEAD2
> done = smp_load_acquire(&rcu_state.srs_done_tail);
> // head = HEAD1
> head = done->next;
> done->next = NULL;
> llist_for_each_safe() {
> // completes all callbacks, release HEAD1
> }
> }
> // Process second queue
> set_work_pool_and_clear_pending()
> rcu_sr_normal_gp_cleanup_work() {
> // done = HEAD2
> done = smp_load_acquire(&rcu_state.srs_done_tail);
>
> // new GP, wait_tail == HEAD3
> rcu_sr_normal_gp_cleanup() {
> // Finds HEAD2 with ->next == NULL at the end
> rcu_sr_put_wait_head(HEAD2)
> ...
>
> // A few more GPs later
> rcu_sr_normal_gp_init() {
> HEAD2 = rcu_sr_get_wait_head();
> llist_add(HEAD2, &rcu_state.srs_next);
> // head == rcu_state.srs_next
> head = done->next;
> done->next = NULL;
> llist_for_each_safe() {
> // EXECUTE CALLBACKS TOO EARLY!!!
> }
> }
Looks like that. To address this, we should not release the head in the GP kthread.
Thanks!
--
Uladzislau Rezki
next prev parent reply other threads:[~2024-02-28 18:04 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-20 18:31 [PATCH v5 0/4] Reduce synchronize_rcu() latency(v5) Uladzislau Rezki (Sony)
2024-02-20 18:31 ` [PATCH v5 1/4] rcu: Add data structures for synchronize_rcu() Uladzislau Rezki (Sony)
2024-02-20 18:31 ` [PATCH v5 2/4] rcu: Reduce synchronize_rcu() latency Uladzislau Rezki (Sony)
2024-02-26 23:07 ` Frederic Weisbecker
2024-02-27 6:39 ` Z qiang
2024-02-27 14:37 ` Frederic Weisbecker
2024-02-27 16:16 ` Frederic Weisbecker
2024-02-27 19:35 ` Uladzislau Rezki
2024-02-28 18:04 ` Uladzislau Rezki [this message]
2024-03-04 11:55 ` Frederic Weisbecker
2024-03-04 16:23 ` Uladzislau Rezki
2024-03-04 20:07 ` Paul E. McKenney
2024-03-05 9:35 ` Uladzislau Rezki
2024-03-04 22:56 ` Frederic Weisbecker
2024-03-05 9:38 ` Uladzislau Rezki
2024-03-05 11:36 ` Frederic Weisbecker
2024-02-27 16:15 ` Frederic Weisbecker
2024-02-27 17:03 ` Frederic Weisbecker
2024-02-27 20:51 ` Joel Fernandes
2024-02-28 9:28 ` Uladzislau Rezki
[not found] ` <4b932245-2825-4e53-87a4-44d2892e7c13@joelfernandes.org>
2024-02-27 22:50 ` Joel Fernandes
2024-02-27 22:53 ` Joel Fernandes
2024-02-28 14:32 ` Joel Fernandes
2024-02-28 16:44 ` Joel Fernandes
2024-02-20 18:31 ` [PATCH v5 3/4] rcu: Add a trace event for synchronize_rcu_normal() Uladzislau Rezki (Sony)
2024-02-20 18:31 ` [PATCH v5 4/4] rcu: Support direct wake-up of synchronize_rcu() users Uladzislau Rezki (Sony)
2024-02-21 1:53 ` [PATCH v5 0/4] Reduce synchronize_rcu() latency(v5) Paul E. McKenney
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