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Message-ID: <d7dfe177-6bfb-63c2-9be5-6ad25c8c243e@huawei.com>
Date: Mon, 29 Jan 2024 08:58:10 +0800
From: Yang Jihong <yangjihong1@...wei.com>
To: <paulmck@...nel.org>
CC: Chen Zhongjin <chenzhongjin@...wei.com>, <linux-kernel@...r.kernel.org>,
<linux-trace-kernel@...r.kernel.org>, <naveen.n.rao@...ux.ibm.com>,
<anil.s.keshavamurthy@...el.com>, <davem@...emloft.net>,
<mhiramat@...nel.org>, <akpm@...ux-foundation.org>, <tglx@...utronix.de>,
<peterz@...radead.org>, <pmladek@...e.com>, <dianders@...omium.org>,
<npiggin@...il.com>, <mpe@...erman.id.au>, <jkl820.git@...il.com>,
<juerg.haefliger@...onical.com>, <rick.p.edgecombe@...el.com>,
<eric.devolder@...cle.com>, <mic@...ikod.net>
Subject: Re: [RFC PATCH] kprobes: Use synchronize_rcu_tasks_rude in
kprobe_optimizer
Hello,
On 2024/1/28 3:22, Paul E. McKenney wrote:
> On Tue, Jan 09, 2024 at 07:28:29PM +0800, Yang Jihong wrote:
>> Hello,
>>
>> PING.
>>
>> I had a similar problem. Is this solution feasible?
>
> Sadly, no.
>
> It fails on CONFIG_PREEMPT=y kernels because synchronize_rcu_tasks_rude()
> will not wait on tasks that have been preempted while executing in
> a trampoline.
>
> But could you please try out the patch shown at the end of this email?
Thanks for the patch, yes, I've tested and it solves the problem.
>
> Thanx, Paul
>
>> Thanks,
>> Yang
>>
>> On 2024/1/2 11:40, Chen Zhongjin wrote:
>>> There is a deadlock scenario in kprobe_optimizer():
>>>
>>> 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
>>>
>>> To avoid this deadlock loop, use synchronize_rcu_tasks_rude() in kprobe_optimizer()
>>> rather than synchronize_rcu_tasks(). synchronize_rcu_tasks_rude() can also promise
>>> that all preempted tasks have scheduled, but it will not wait tasks_rcu_exit_srcu.
>>>
>>> Signed-off-by: Chen Zhongjin <chenzhongjin@...wei.com>
>>> ---
>>> arch/Kconfig | 2 +-
>>> kernel/kprobes.c | 2 +-
>>> 2 files changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/Kconfig b/arch/Kconfig
>>> index f4b210ab0612..dc6a18854017 100644
>>> --- a/arch/Kconfig
>>> +++ b/arch/Kconfig
>>> @@ -104,7 +104,7 @@ config STATIC_CALL_SELFTEST
>>> config OPTPROBES
>>> def_bool y
>>> depends on KPROBES && HAVE_OPTPROBES
>>> - select TASKS_RCU if PREEMPTION
>>> + select TASKS_RUDE_RCU
>>> config KPROBES_ON_FTRACE
>>> def_bool y
>>> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
>>> index d5a0ee40bf66..09056ae50c58 100644
>>> --- a/kernel/kprobes.c
>>> +++ b/kernel/kprobes.c
>>> @@ -623,7 +623,7 @@ static void kprobe_optimizer(struct work_struct *work)
>>> * Note that on non-preemptive kernel, this is transparently converted
>>> * to synchronoze_sched() to wait for all interrupts to have completed.
>>> */
>>> - synchronize_rcu_tasks();
>>> + synchronize_rcu_tasks_rude();
>>> /* Step 3: Optimize kprobes after quiesence period */
>>> do_optimize_kprobes();
>
> ------------------------------------------------------------------------
>
> commit bc31e6cb27a9334140ff2f0a209d59b08bc0bc8c
> 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 cdb8ea53c365..4f0e9274da2d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -858,6 +858,8 @@ 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 7ecb458eb3da..4daee6d761c8 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -147,6 +147,7 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = {
> .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 47ff3b35352e..3eb86f30e664 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1975,6 +1975,7 @@ 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);
> }
>
> /*
>
Tested-by: Yang Jihong <yangjihong1@...wei.com>
Thanks,
Yang
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