On Thu, Jul 6, 2023 at 9:37 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote: > > Hi, > > On 7/7/2023 12:16 PM, Alexei Starovoitov wrote: > > On Thu, Jul 6, 2023 at 8:39 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote: > >> Hi, > >> > >> On 7/7/2023 10:12 AM, Alexei Starovoitov wrote: > >>> On Thu, Jul 6, 2023 at 7:07 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote: > >>>> Hi, > >>>> > >>>> On 7/6/2023 11:34 AM, Alexei Starovoitov wrote: > >>>> > SNIP > >>> and it's not just waiting_for_gp_ttrace. free_by_rcu_ttrace is similar. > >> I think free_by_rcu_ttrace is different, because the reuse is only > >> possible after one tasks trace RCU grace period as shown below, and the > >> concurrent llist_del_first() must have been completed when the head is > >> reused and re-added into free_by_rcu_ttrace again. > >> > >> // c0->free_by_rcu_ttrace > >> A -> B -> C -> nil > >> > >> P1: > >> alloc_bulk() > >> llist_del_first(&c->free_by_rcu_ttrace) > >> entry = A > >> next = B > >> > >> P2: > >> do_call_rcu_ttrace() > >> // c->free_by_rcu_ttrace->first = NULL > >> llist_del_all(&c->free_by_rcu_ttrace) > >> move to c->waiting_for_gp_ttrace > >> > >> P1: > >> llist_del_first() > >> return NULL > >> > >> // A is only reusable after one task trace RCU grace > >> // llist_del_first() must have been completed > > "must have been completed" ? > > > > I guess you're assuming that alloc_bulk() from irq_work > > is running within rcu_tasks_trace critical section, > > so __free_rcu_tasks_trace() callback will execute after > > irq work completed? > > I don't think that's the case. > > Yes. The following is my original thoughts. Correct me if I was wrong: > > 1. llist_del_first() must be running concurrently with llist_del_all(). > If llist_del_first() runs after llist_del_all(), it will return NULL > directly. > 2. call_rcu_tasks_trace() must happen after llist_del_all(), else the > elements in free_by_rcu_ttrace will not be freed back to slab. > 3. call_rcu_tasks_trace() will wait for one tasks trace RCU grace period > to call __free_rcu_tasks_trace() > 4. llist_del_first() in running in an context with irq-disabled, so the > tasks trace RCU grace period will wait for the end of llist_del_first() > > It seems you thought step 4) is not true, right ? Yes. I think so. For two reasons: 1. I believe irq disabled region isn't considered equivalent to rcu_read_lock_trace() region. Paul, could you clarify ? 2. Even if 1 is incorrect, in RT llist_del_first() from alloc_bulk() runs "in a per-CPU thread in preemptible context." See irq_work_run_list.
