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

[Frederic Weisbecker]

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

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>