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Message-ID: <14ab5bd4-d7b8-4233-9389-f21884986671@joelfernandes.org>
Date: Thu, 7 Mar 2024 01:21:50 -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 3/6/2024 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).
Could be something like this, but maybe I missed something:
@@ -1672,7 +1674,7 @@ static void rcu_sr_normal_gp_cleanup_work(struct
work_struct *work)
*/
static void rcu_sr_normal_gp_cleanup(void)
{
- struct llist_node *wait_tail, *next, *rcu;
+ struct llist_node *wait_tail, *next = NULL, *rcu = NULL;
int done = 0;
wait_tail = rcu_state.srs_wait_tail;
@@ -1707,7 +1709,8 @@ static void rcu_sr_normal_gp_cleanup(void)
* 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);
+ if (rcu)
+ queue_work(system_highpri_wq, &rcu_state.srs_cleanup_work);
}
/*
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