linux-kernel - Re: [PATCH v5 2/4] rcu: Reduce synchronize

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Date: Tue, 27 Feb 2024 15:37:26 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Z qiang <qiang.zhang1211@...il.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
	"Paul E . McKenney" <paulmck@...nel.org>, RCU <rcu@...r.kernel.org>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v5 2/4] rcu: Reduce synchronize_rcu() latency

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...