On Sat, Jan 27, 2024 at 06:09:05PM +0800, Chen Zhongjin wrote:
> On 2024/1/20 23:30, Paul E. McKenney wrote:
(Apologies for the delay, despite my attempts to make it otherwise,
your email still got dumped into my spam folder.)
> Hi Paul,
> This patch works for my reproduce test case.
Thank you!!!
> Just a small question, if you dont mind, this problem exsit on LTS version
> but we had struct rcu_tasks_percpu after v5.17. We can't backport this patch
> to LTS 5.10 or 4.19, which are still under maintaince.
> If you have any idea or plan to apply this patch to elder version, please
> tell me, thanks very much!
It should be possible to hand-apply the patch. Or to backport additional
patches to make this one apply cleanly. Note that in v4.19, the code
is in kernel/rcu/update.c rather than its new home in kernel/rcu/tasks.h.
> Anyway, it's ok to apply this patch to mainline.
May I have your Tested-by?
Thanx, Paul
> Best,
> Chen
>
> > > Again, that comment reads in full as follows:
> > >
> > > /*
> > > * Step 2: Wait for quiesence period to ensure all potentially
> > > * preempted tasks to have normally scheduled. Because optprobe
> > > * may modify multiple instructions, there is a chance that Nth
> > > * instruction is preempted. In that case, such tasks can return
> > > * to 2nd-Nth byte of jump instruction. This wait is for avoiding it.
> > > * Note that on non-preemptive kernel, this is transparently converted
> > > * to synchronoze_sched() to wait for all interrupts to have completed.
> > > */
> > >
> > > Please note well that first sentence.
> > >
> > > Unless that first sentence no longer holds, this patch cannot work
> > > because synchronize_rcu_tasks_rude() will not (repeat, NOT) wait for
> > > preempted tasks.
> > >
> > > So how to safely break this deadlock? Reproducing Chen Zhongjin's
> > > diagram:
> > >
> > > pid A pid B pid C
> > > kprobe_optimizer() do_exit()
> > > perf_kprobe_init()
> > > mutex_lock(&kprobe_mutex) exit_tasks_rcu_start()
> > > mutex_lock(&kprobe_mutex)
> > > synchronize_rcu_tasks() zap_pid_ns_processes() // waiting
> > > kprobe_mutex
> > > // waiting tasks_rcu_exit_srcu kernel_wait4()
> > > // waiting pid C exit
> > >
> > > We need to stop synchronize_rcu_tasks() from waiting on tasks like
> > > pid B that are voluntarily blocked. One way to do that is to replace
> > > SRCU with a set of per-CPU lists. Then exit_tasks_rcu_start() adds the
> > > current task to this list and does ...
> > >
> > > OK, this is getting a bit involved. If you would like to follow along,
> > > please feel free to look here:
> > >
> > > https://docs.google.com/document/d/1MEHHs5qbbZBzhN8dGP17pt-d87WptFJ2ZQcqS221d9I/edit?usp=sharing
> >
> > And please see below for a prototype patch, which passes moderate
> > rcutorture testing.
> >
> > Chen Zhongjin, does this prevent the deadlock you have been seeing?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit 113fe0eeabe7a83e87d638d44b9e1d0f9691b146
> > Author: Paul E. McKenney <paul...@kernel.org>
> > Date: Sat Jan 20 07:07:08 2024 -0800
> >
> > rcu-tasks: Eliminate deadlocks involving do_exit() and RCU tasks
> > 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-chenzhong...@huawei.com/
> > Reported-by: Chen Zhongjin <chenzhong...@huawei.com>
> > Signed-off-by: Paul E. McKenney <paul...@kernel.org>
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index dddd10b1b815..3fe36befb613 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -855,6 +855,8 @@ struct task_struct {
> > u8 rcu_tasks_idx;
> > int rcu_tasks_idle_cpu;
> > struct list_head rcu_tasks_holdout_list;
> > + int rcu_tasks_exit_cpu;
> > + 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 5727d42149c3..65f037bff457 100644
> > --- a/init/init_task.c
> > +++ b/init/init_task.c
> > @@ -152,6 +152,7 @@ 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 10917c3e1f03..6bacd515e0eb 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -1981,6 +1981,7 @@ 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 732ad5b39946..bd4a51fd5b1f 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -32,6 +32,7 @@ typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
> > * @rtp_irq_work: IRQ work queue for deferred wakeups.
> > * @barrier_q_head: RCU callback for barrier operation.
> > * @rtp_blkd_tasks: List of tasks blocked as readers.
> > + * @rtp_exit_list: List of tasks in the latter portion of do_exit().
> > * @cpu: CPU number corresponding to this entry.
> > * @rtpp: Pointer to the rcu_tasks structure.
> > */
> > @@ -46,6 +47,7 @@ struct rcu_tasks_percpu {
> > struct irq_work rtp_irq_work;
> > struct rcu_head barrier_q_head;
> > struct list_head rtp_blkd_tasks;
> > + struct list_head rtp_exit_list;
> > int cpu;
> > struct rcu_tasks *rtpp;
> > };
> > @@ -144,8 +146,6 @@ static struct rcu_tasks rt_name =
> > \
> > }
> > #ifdef CONFIG_TASKS_RCU
> > -/* Track exiting tasks in order to allow them to be waited for. */
> > -DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
> > /* Report delay in synchronize_srcu() completion in rcu_tasks_postscan().
> > */
> > static void tasks_rcu_exit_srcu_stall(struct timer_list *unused);
> > @@ -275,6 +275,8 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
> > rtpcp->rtpp = rtp;
> > if (!rtpcp->rtp_blkd_tasks.next)
> > INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
> > + if (!rtpcp->rtp_exit_list.next)
> > + INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
> > }
> > pr_info("%s: Setting shift to %d and lim to %d
> > rcu_task_cb_adjust=%d.\n", rtp->name,
> > @@ -851,10 +853,12 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
> > // number of voluntary context switches, and add that task to the
> > // holdout list.
> > // rcu_tasks_postscan():
> > -// Invoke synchronize_srcu() to ensure that all tasks that were
> > -// in the process of exiting (and which thus might not know to
> > -// synchronize with this RCU Tasks grace period) have completed
> > -// exiting.
> > +// Gather per-CPU lists of tasks in do_exit() to ensure that all
> > +// tasks that were in the process of exiting (and which thus might
> > +// not know to synchronize with this RCU Tasks grace period) have
> > +// completed exiting. The synchronize_rcu() in rcu_tasks_postgp()
> > +// will take care of any tasks stuck in the non-preemptible region
> > +// of do_exit() following its call to exit_tasks_rcu_stop().
> > // check_all_holdout_tasks(), repeatedly until holdout list is empty:
> > // Scans the holdout list, attempting to identify a quiescent state
> > // for each task on the list. If there is a quiescent state, the
> > @@ -867,8 +871,10 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
> > // with interrupts disabled.
> > //
> > // For each exiting task, the exit_tasks_rcu_start() and
> > -// exit_tasks_rcu_finish() functions begin and end, respectively, the SRCU
> > -// read-side critical sections waited for by rcu_tasks_postscan().
> > +// exit_tasks_rcu_finish() functions add and remove, respectively, the
> > +// current task to a per-CPU list of tasks that rcu_tasks_postscan() must
> > +// wait on. This is necessary because rcu_tasks_postscan() must wait on
> > +// tasks that have already been removed from the global list of tasks.
> > //
> > // Pre-grace-period update-side code is ordered before the grace
> > // via the raw_spin_lock.*rcu_node(). Pre-grace-period read-side code
> > @@ -932,9 +938,13 @@ static void rcu_tasks_pertask(struct task_struct *t,
> > struct list_head *hop)
> > }
> > }
> > +void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
> > +DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU
> > Tasks");
> > +
> > /* Processing between scanning taskslist and draining the holdout list. */
> > static void rcu_tasks_postscan(struct list_head *hop)
> > {
> > + int cpu;
> > int rtsi = READ_ONCE(rcu_task_stall_info);
> > if (!IS_ENABLED(CONFIG_TINY_RCU)) {
> > @@ -948,9 +958,9 @@ static void rcu_tasks_postscan(struct list_head *hop)
> > * 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()
> > @@ -959,7 +969,18 @@ 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);
> > +
> > + for_each_possible_cpu(cpu) {
> > + unsigned long flags;
> > + struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu,
> > cpu);
> > + struct task_struct *t;
> > +
> > + raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
> > + list_for_each_entry(t, &rtpcp->rtp_exit_list,
> > rcu_tasks_exit_list)
> > + if (list_empty(&t->rcu_tasks_holdout_list))
> > + rcu_tasks_pertask(t, hop);
> > + raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
> > + }
> > if (!IS_ENABLED(CONFIG_TINY_RCU))
> > del_timer_sync(&tasks_rcu_exit_srcu_stall_timer);
> > @@ -1027,7 +1048,6 @@ 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.
> > @@ -1035,9 +1055,6 @@ static void rcu_tasks_postgp(struct rcu_tasks *rtp)
> > synchronize_rcu();
> > }
> > -void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
> > -DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU
> > Tasks");
> > -
> > static void tasks_rcu_exit_srcu_stall(struct timer_list *unused)
> > {
> > #ifndef CONFIG_TINY_RCU
> > @@ -1147,25 +1164,45 @@ struct task_struct *get_rcu_tasks_gp_kthread(void)
> > EXPORT_SYMBOL_GPL(get_rcu_tasks_gp_kthread);
> > /*
> > - * 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 rcu_tasks_percpu *rtpcp;
> > + struct task_struct *t = current;
> > +
> > + WARN_ON_ONCE(!list_empty(&t->rcu_tasks_exit_list));
> > + get_task_struct(t);
> > + 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);
> > + 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);
> > }
> > /*
