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@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>
>
Thank you, I will update with your review-by tag!
--
Uladzislau Rezki
