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 01:04:25AM +0100, Frederic Weisbecker wrote:
> Le Tue, Mar 05, 2024 at 08:57:19PM +0100, Uladzislau Rezki (Sony) a écrit :
> > 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>
> 
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
>
Thank you, I will update with your review-by tag!

--
Uladzislau Rezki