From: Frederic Weisbecker <frederic@kernel.org>
To: Z qiang <qiang.zhang1211@gmail.com>
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: Tue, 27 Feb 2024 17:16:27 +0100 [thread overview]
Message-ID: <Zd4K28_GkRQBn9Ik@localhost.localdomain> (raw)
In-Reply-To: <Zd3zptTLlUDCg1L7@localhost.localdomain>
Le Tue, Feb 27, 2024 at 03:37:26PM +0100, Frederic Weisbecker a écrit :
> Le Tue, Feb 27, 2024 at 02:39:55PM +0800, Z qiang a écrit :
> > > 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)
> >
> > It seems that we should move rcu_sr_put_wait_head() from
> > rcu_sr_normal_gp_cleanup() to
> > rcu_sr_normal_gp_cleanup_work(), if find wait_head->next == NULL, invoke
> > rcu_sr_put_wait_head() to release wait_head.
>
> Well, rcu_sr_normal_gp_cleanup_work() already put all the wait heads
> that are _after_ srs_done_tail. But it can't put the srs_done_tail itself
> without introducing even worse races...
>
(I forgot to mention this race actually concerns the last patch (4/4))
next prev parent reply other threads:[~2024-02-27 16:16 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 [this message]
2024-02-27 19:35 ` Uladzislau Rezki
2024-02-28 18:04 ` Uladzislau Rezki
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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