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>
