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Message-ID: <648f31c5-e67c-4605-9ebd-7450e96a2dbe@joelfernandes.org>
Date: Thu, 7 Mar 2024 02:09:38 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
"Paul E . McKenney" <paulmck@...nel.org>
Cc: 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 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).
thanks,
- Joel
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