Message-ID: <526fbe50-49a0-439c-8f3d-9559b7e1d737@paulmck-laptop>
Date: Thu, 1 Feb 2024 05:47:23 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Chen Zhongjin <chenzhongjin@...wei.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-trace-kernel@...r.kernel.org" <linux-trace-kernel@...r.kernel.org>,
	yangjihong <yangjihong1@...wei.com>,
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Subject: Re: [PATCH v2] kprobes: Use synchronize_rcu_tasks_rude in
 kprobe_optimizer

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 <paulmck@...nel.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-chenzhongjin@huawei.com/
> >      Reported-by: Chen Zhongjin <chenzhongjin@...wei.com>
> >      Signed-off-by: Paul E. McKenney <paulmck@...nel.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);
> >   }
> >   /*

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