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

[Uladzislau Rezki <urezki@gmail.com>]

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@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).
> 
No problem at all :)

--
Uladzislau Rezki