Re: [PATCH 1/2] rcu: Do not release a wait-head from a GP kthread

[Joel Fernandes <joel@joelfernandes.org>]

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@kernel.org>
>>> Fixes: 05a10b921000 ("rcu: Support direct wake-up of synchronize_rcu() users")
>>> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.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