Le Wed, Feb 07, 2024 at 11:53:13PM +0100, Frederic Weisbecker a écrit : > 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() ? Or maybe it orders add into rtpcp->rtp_exit_list VS main tasklist's removal? Such that: synchronize_rcu_tasks() do_exit() ---------------------- --------- //for_each_process_thread() READ tasklist WRITE rtpcp->rtp_exit_list LOCK rtpcp->lock UNLOCK rtpcp->lock smp_mb__after_unlock_lock() WRITE tasklist //unhash_process() READ rtpcp->rtp_exit_list Does this work? Hmm, I'll play with litmus once I have a fresh brain... Thanks.
