From: Frederic Weisbecker <frederic@kernel.org>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: "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: Mon, 4 Mar 2024 12:55:47 +0100 [thread overview]
Message-ID: <ZeW2w08WZo4yapQp@localhost.localdomain> (raw)
In-Reply-To: <Zd91pR0fjiCUZTtP@pc636>
Le Wed, Feb 28, 2024 at 07:04:21PM +0100, Uladzislau Rezki a écrit :
> 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.
But then you have to unconditionally schedule the work, right? Otherwise the
HEADs are not released. And that means dropping this patch (right now I don't
have a better idea).
next prev parent reply other threads:[~2024-03-04 11:55 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
2024-03-04 11:55 ` Frederic Weisbecker [this message]
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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