Le Fri, Feb 16, 2024 at 05:27:40PM -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> > Reported-by: Yang Jihong <yangjihong1@xxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > Tested-by: Yang Jihong <yangjihong1@xxxxxxxxxx> > Tested-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx> > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx> Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
