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Message-ID: <ZemzrXfN7PUGiETd@pc636>
Date: Thu, 7 Mar 2024 13:31:41 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>
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>,
LKML <linux-kernel@...r.kernel.org>,
Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH 1/2] rcu: Do not release a wait-head from a GP kthread
On Thu, Mar 07, 2024 at 02:09:38AM -0500, Joel Fernandes wrote:
>
>
> On 3/7/2024 1:21 AM, Joel Fernandes wrote:
> >
> >
> > On 3/6/2024 5:31 PM, Joel Fernandes wrote:
> >>
> >>
> >> On 3/5/2024 2:57 PM, Uladzislau Rezki (Sony) wrote:
> >>> Fix a below race by not releasing a wait-head from the
> >>> GP-kthread as it can lead for reusing it whereas a worker
> >>> can still access it thus execute newly added callbacks too
> >>> early.
> >>>
> >>> 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!!!
> >>> }
> >>> }
> >>>
> >>> Reported-by: Frederic Weisbecker <frederic@...nel.org>
> >>> Fixes: 05a10b921000 ("rcu: Support direct wake-up of synchronize_rcu() users")
> >>> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> >>> ---
> >>> kernel/rcu/tree.c | 22 ++++++++--------------
> >>> 1 file changed, 8 insertions(+), 14 deletions(-)
> >>>
> >>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >>> index 31f3a61f9c38..475647620b12 100644
> >>> --- a/kernel/rcu/tree.c
> >>> +++ b/kernel/rcu/tree.c
> >>> @@ -1656,21 +1656,11 @@ static void rcu_sr_normal_gp_cleanup(void)
> >>> WARN_ON_ONCE(!rcu_sr_is_wait_head(wait_tail));
> >>>
> >>> /*
> >>> - * Process (a) and (d) cases. See an illustration. Apart of
> >>> - * that it handles the scenario when all clients are done,
> >>> - * wait-head is released if last. The worker is not kicked.
> >>> + * Process (a) and (d) cases. See an illustration.
> >>> */
> >>> llist_for_each_safe(rcu, next, wait_tail->next) {
> >>> - if (rcu_sr_is_wait_head(rcu)) {
> >>> - if (!rcu->next) {
> >>> - rcu_sr_put_wait_head(rcu);
> >>> - wait_tail->next = NULL;
> >>> - } else {
> >>> - wait_tail->next = rcu;
> >>> - }
> >>> -
> >>> + if (rcu_sr_is_wait_head(rcu))
> >>> break;
> >>> - }
> >>>
> >>> rcu_sr_normal_complete(rcu);
> >>> // It can be last, update a next on this step.
> >>> @@ -1684,8 +1674,12 @@ static void rcu_sr_normal_gp_cleanup(void)
> >>> smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> >>> ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
> >>>
> >>> - if (wait_tail->next)
> >>> - queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
> >>> + /*
> >>> + * We schedule a work in order to perform a final processing
> >>> + * of outstanding users(if still left) and releasing wait-heads
> >>> + * added by rcu_sr_normal_gp_init() call.
> >>> + */
> >>> + queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
> >>> }
> >
> > One question, why do you need to queue_work() if wait_tail->next == NULL?
> >
> > AFAICS, at this stage if wait_tail->next == NULL, you are in CASE f. so the last
> > remaining HEAD stays? (And llist_for_each_safe() in
> > rcu_sr_normal_gp_cleanup_work becomes a NOOP).
> >
>
> Never mind, sorry for spewing nonsense. You can never end up with CASE f here so
> you still need the queue_work(). I think it is worth looking into how to free
> the last HEAD, without queuing a work though (while not causing wreckage).
>
No problem at all :)
--
Uladzislau Rezki
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