Le Mon, Jan 29, 2024 at 02:57:27PM -0800, Boqun Feng a écrit : > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > Holding a mutex across synchronize_rcu_tasks() and acquiring > that same mutex in code called from do_exit() after its call to > exit_tasks_rcu_start() but before its call to exit_tasks_rcu_stop() > results in deadlock. This is by design, because tasks that are far > enough into do_exit() are no longer present on the tasks list, making > it a bit difficult for RCU Tasks to find them, let alone wait on them > to do a voluntary context switch. However, such deadlocks are becoming > more frequent. In addition, lockdep currently does not detect such > deadlocks and they can be difficult to reproduce. > > In addition, if a task voluntarily context switches during that time > (for example, if it blocks acquiring a mutex), then this task is in an > RCU Tasks quiescent state. And with some adjustments, RCU Tasks could > just as well take advantage of that fact. > > This commit therefore eliminates these deadlock by replacing the > SRCU-based wait for do_exit() completion with per-CPU lists of tasks > currently exiting. A given task will be on one of these per-CPU lists for > the same period of time that this task would previously have been in the > previous SRCU read-side critical section. These lists enable RCU Tasks > to find the tasks that have already been removed from the tasks list, > but that must nevertheless be waited upon. > > The RCU Tasks grace period gathers any of these do_exit() tasks that it > must wait on, and adds them to the list of holdouts. Per-CPU locking > and get_task_struct() are used to synchronize addition to and removal > from these lists. > > Link: https://lore.kernel.org/all/20240118021842.290665-1-chenzhongjin@xxxxxxxxxx/ > > Reported-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> With that, I think we can now revert 28319d6dc5e2 (rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()). Because if the task is in rcu_tasks_exit_list, it's treated just like the others and must go through check_holdout_task(). Therefore and unlike with the previous srcu thing, a task sleeping between exit_tasks_rcu_start() and exit_tasks_rcu_finish() is now a quiescent state. And that kills the possible deadlock. > -void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu) > +void exit_tasks_rcu_start(void) > { > - current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu); > + unsigned long flags; > + struct rcu_tasks_percpu *rtpcp; > + struct task_struct *t = current; > + > + WARN_ON_ONCE(!list_empty(&t->rcu_tasks_exit_list)); > + get_task_struct(t); Is this get_task_struct() necessary? > + preempt_disable(); > + rtpcp = this_cpu_ptr(rcu_tasks.rtpcpu); > + t->rcu_tasks_exit_cpu = smp_processor_id(); > + raw_spin_lock_irqsave_rcu_node(rtpcp, flags); Do we really need smp_mb__after_unlock_lock() ? > + if (!rtpcp->rtp_exit_list.next) > + INIT_LIST_HEAD(&rtpcp->rtp_exit_list); > + list_add(&t->rcu_tasks_exit_list, &rtpcp->rtp_exit_list); > + raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags); > + preempt_enable(); > } > > /* > - * Contribute to protect against tasklist scan blind spot while the > - * task is exiting and may be removed from the tasklist. See > - * corresponding synchronize_srcu() for further details. > + * Remove the task from the "yet another list" because do_exit() is now > + * non-preemptible, allowing synchronize_rcu() to wait beyond this point. > */ > -void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu) > +void exit_tasks_rcu_stop(void) > { > + unsigned long flags; > + struct rcu_tasks_percpu *rtpcp; > struct task_struct *t = current; > > - __srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx); > + WARN_ON_ONCE(list_empty(&t->rcu_tasks_exit_list)); > + rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, t->rcu_tasks_exit_cpu); > + raw_spin_lock_irqsave_rcu_node(rtpcp, flags); > + list_del_init(&t->rcu_tasks_exit_list); > + raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags); > + put_task_struct(t); And conversely this put_task_struct()? Thanks. > } > > /* > -- > 2.43.0 >
