From: Paul E. McKenney <paulmck@xxxxxxxxxx> commit bc31e6cb27a9334140ff2f0a209d59b08bc0bc8c upstream. 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> Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx> --- include/linux/sched.h | 1 + init/init_task.c | 1 + kernel/fork.c | 1 + kernel/rcu/tasks.h | 54 ++++++++++++++++++++++++++++--------------- 4 files changed, 39 insertions(+), 18 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index aa015416c569..80499f7ab39a 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -740,6 +740,7 @@ struct task_struct { u8 rcu_tasks_idx; int rcu_tasks_idle_cpu; struct list_head rcu_tasks_holdout_list; + struct list_head rcu_tasks_exit_list; #endif /* #ifdef CONFIG_TASKS_RCU */ #ifdef CONFIG_TASKS_TRACE_RCU diff --git a/init/init_task.c b/init/init_task.c index 5fa18ed59d33..59454d6e2c2a 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -151,6 +151,7 @@ struct task_struct init_task .rcu_tasks_holdout = false, .rcu_tasks_holdout_list = LIST_HEAD_INIT(init_task.rcu_tasks_holdout_list), .rcu_tasks_idle_cpu = -1, + .rcu_tasks_exit_list = LIST_HEAD_INIT(init_task.rcu_tasks_exit_list), #endif #ifdef CONFIG_TASKS_TRACE_RCU .trc_reader_nesting = 0, diff --git a/kernel/fork.c b/kernel/fork.c index 633b0af1d1a7..86803165aa00 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1699,6 +1699,7 @@ static inline void rcu_copy_process(struct task_struct *p) p->rcu_tasks_holdout = false; INIT_LIST_HEAD(&p->rcu_tasks_holdout_list); p->rcu_tasks_idle_cpu = -1; + INIT_LIST_HEAD(&p->rcu_tasks_exit_list); #endif /* #ifdef CONFIG_TASKS_RCU */ #ifdef CONFIG_TASKS_TRACE_RCU p->trc_reader_nesting = 0; diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index c5624ab0580c..901cd7bc78ed 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -81,9 +81,6 @@ static struct rcu_tasks rt_name = \ .kname = #rt_name, \ } -/* Track exiting tasks in order to allow them to be waited for. */ -DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu); - /* Avoid IPIing CPUs early in the grace period. */ #define RCU_TASK_IPI_DELAY (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) ? HZ / 2 : 0) static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY; @@ -383,6 +380,9 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp) // rates from multiple CPUs. If this is required, per-CPU callback lists // will be needed. +static LIST_HEAD(rtp_exit_list); +static DEFINE_RAW_SPINLOCK(rtp_exit_list_lock); + /* Pre-grace-period preparation. */ static void rcu_tasks_pregp_step(void) { @@ -416,15 +416,18 @@ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop) /* Processing between scanning taskslist and draining the holdout list. */ static void rcu_tasks_postscan(struct list_head *hop) { + unsigned long flags; + struct task_struct *t; + /* * Exiting tasks may escape the tasklist scan. Those are vulnerable * until their final schedule() with TASK_DEAD state. To cope with * this, divide the fragile exit path part in two intersecting * read side critical sections: * - * 1) An _SRCU_ read side starting before calling exit_notify(), - * which may remove the task from the tasklist, and ending after - * the final preempt_disable() call in do_exit(). + * 1) A task_struct list addition before calling exit_notify(), + * which may remove the task from the tasklist, with the + * removal after the final preempt_disable() call in do_exit(). * * 2) An _RCU_ read side starting with the final preempt_disable() * call in do_exit() and ending with the final call to schedule() @@ -433,7 +436,12 @@ static void rcu_tasks_postscan(struct list_head *hop) * This handles the part 1). And postgp will handle part 2) with a * call to synchronize_rcu(). */ - synchronize_srcu(&tasks_rcu_exit_srcu); + raw_spin_lock_irqsave(&rtp_exit_list_lock, flags); + list_for_each_entry(t, &rtp_exit_list, rcu_tasks_exit_list) { + if (list_empty(&t->rcu_tasks_holdout_list)) + rcu_tasks_pertask(t, hop); + } + raw_spin_unlock_irqrestore(&rtp_exit_list_lock, flags); } /* See if tasks are still holding out, complain if so. */ @@ -498,7 +506,6 @@ static void rcu_tasks_postgp(struct rcu_tasks *rtp) * * In addition, this synchronize_rcu() waits for exiting tasks * to complete their final preempt_disable() region of execution, - * cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu), * enforcing the whole region before tasklist removal until * the final schedule() with TASK_DEAD state to be an RCU TASKS * read side critical section. @@ -591,25 +598,36 @@ static void show_rcu_tasks_classic_gp_kthread(void) #endif /* #ifndef CONFIG_TINY_RCU */ /* - * 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. + * Protect against tasklist scan blind spot while the task is exiting and + * may be removed from the tasklist. Do this by adding the task to yet + * another list. */ -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 task_struct *t = current; + + WARN_ON_ONCE(!list_empty(&t->rcu_tasks_exit_list)); + get_task_struct(t); + raw_spin_lock_irqsave(&rtp_exit_list_lock, flags); + list_add(&t->rcu_tasks_exit_list, &rtp_exit_list); + raw_spin_unlock_irqrestore(&rtp_exit_list_lock, flags); } /* - * 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 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)); + raw_spin_lock_irqsave(&rtp_exit_list_lock, flags); + list_del_init(&t->rcu_tasks_exit_list); + raw_spin_unlock_irqrestore(&rtp_exit_list_lock, flags); + put_task_struct(t); } /* -- 2.17.1
