Re: [PATCH v2 3/6] rcu-tasks: Initialize data to eliminate RCU-tasks/do_exit() deadlocks

Paul E. McKenney
2024-02-22T20:41:56Z

On Thu, Feb 22, 2024 at 05:21:03PM +0100, Frederic Weisbecker wrote:
> Le Fri, Feb 16, 2024 at 05:27:38PM -0800, Boqun Feng a écrit :
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> >
> > 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 initializes the data structures that will be needed
> > to rely on these quiescent states and to eliminate these deadlocks.
> >
> > Link: https://lore.kernel.org/all/20240118021842.290665-1-chenzhongjin@huawei.com/
> >
> > Reported-by: Chen Zhongjin <chenzhongjin@huawei.com>
> > Reported-by: Yang Jihong <yangjihong1@huawei.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Tested-by: Yang Jihong <yangjihong1@huawei.com>
> > Tested-by: Chen Zhongjin <chenzhongjin@huawei.com>
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> > init/init_task.c | 1 +
> > kernel/fork.c | 1 +
> > kernel/rcu/tasks.h | 2 ++
> > 3 files changed, 4 insertions(+)
> >
> > 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 0d944e92a43f..af7203be1d2d 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -1976,6 +1976,7 @@ 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 b7d5f2757053..4a5d562e3189 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -277,6 +277,8 @@ 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)
>
> I assume there can't be an exiting task concurrently at this point on
> boot. Because kthreadd just got created and workqueues as well but that's it,
> right? Or workqueues can die that early? Probably not.
>
> > + INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
>
> Because if tasks can exit concurrently, then we are in trouble :-)

Tasks exiting at that point might be unconventional, but I don't see
anything that prevents them from doing so.

So excellent catch, and thank you very much!!!

My thought is to add the following patch to precede this one, which
initializes those lists at rcu_init() time. Would that work?

Thanx, Paul

------------------------------------------------------------------------

commit 9a876aac8064dfd46c840e4bb6177e65f7964bb4
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Thu Feb 22 12:29:54 2024 -0800

rcu-tasks: Initialize callback lists at rcu_init() time

In order for RCU Tasks to reliably maintain per-CPU lists of exiting
tasks, those lists must be initialized before it is possible for tasks
to exit, especially given that the boot CPU is not necessarily CPU 0
(an example being, powerpc kexec() kernels). And at the time that
rcu_init_tasks_generic() is called, a task could potentially exit,
unconventional though that sort of thing might be.

This commit therefore moves the calls to cblist_init_generic() from
functions called from rcu_init_tasks_generic() to a new function named
tasks_cblist_init_generic() that is invoked from rcu_init().

This constituted a bug in a commit that never went to mainline, so
there is no need for any backporting to -stable.

Reported-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 4e65a92e528e5..86fce206560e8 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -528,6 +528,12 @@ struct task_struct *get_rcu_tasks_gp_kthread(void);
struct task_struct *get_rcu_tasks_rude_gp_kthread(void);
#endif // # ifdef CONFIG_TASKS_RUDE_RCU

+#ifdef CONFIG_TASKS_RCU_GENERIC
+void tasks_cblist_init_generic(void);
+#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
+static inline void tasks_cblist_init_generic(void) { }
+#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
+
#define RCU_SCHEDULER_INACTIVE 0
#define RCU_SCHEDULER_INIT 1
#define RCU_SCHEDULER_RUNNING 2
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 866743e0796f4..e06e388e7c7e6 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -240,7 +240,6 @@ static const char *tasks_gp_state_getname(struct rcu_tasks *rtp)
static void cblist_init_generic(struct rcu_tasks *rtp)
{
int cpu;
- unsigned long flags;
int lim;
int shift;

@@ -266,10 +265,8 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
WARN_ON_ONCE(!rtpcp);
if (cpu)
raw_spin_lock_init(&ACCESS_PRIVATE(rtpcp, lock));
- local_irq_save(flags); // serialize initialization
if (rcu_segcblist_empty(&rtpcp->cblist))
rcu_segcblist_init(&rtpcp->cblist);
- local_irq_restore(flags);
INIT_WORK(&rtpcp->rtp_work, rcu_tasks_invoke_cbs_wq);
rtpcp->cpu = cpu;
rtpcp->rtpp = rtp;
@@ -1153,7 +1150,6 @@ module_param(rcu_tasks_lazy_ms, int, 0444);

static int __init rcu_spawn_tasks_kthread(void)
{
- cblist_init_generic(&rcu_tasks);
rcu_tasks.gp_sleep = HZ / 10;
rcu_tasks.init_fract = HZ / 10;
if (rcu_tasks_lazy_ms >= 0)
@@ -1340,7 +1336,6 @@ module_param(rcu_tasks_rude_lazy_ms, int, 0444);

static int __init rcu_spawn_tasks_rude_kthread(void)
{
- cblist_init_generic(&rcu_tasks_rude);
rcu_tasks_rude.gp_sleep = HZ / 10;
if (rcu_tasks_rude_lazy_ms >= 0)
rcu_tasks_rude.lazy_jiffies = msecs_to_jiffies(rcu_tasks_rude_lazy_ms);
@@ -1972,7 +1967,6 @@ module_param(rcu_tasks_trace_lazy_ms, int, 0444);

static int __init rcu_spawn_tasks_trace_kthread(void)
{
- cblist_init_generic(&rcu_tasks_trace);
if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) {
rcu_tasks_trace.gp_sleep = HZ / 10;
rcu_tasks_trace.init_fract = HZ / 10;
@@ -2144,6 +2138,24 @@ late_initcall(rcu_tasks_verify_schedule_work);
static void rcu_tasks_initiate_self_tests(void) { }
#endif /* #else #ifdef CONFIG_PROVE_RCU */

+void __init tasks_cblist_init_generic(void)
+{
+ lockdep_assert_irqs_disabled();
+ WARN_ON(num_online_cpus() > 1);
+
+#ifdef CONFIG_TASKS_RCU
+ cblist_init_generic(&rcu_tasks);
+#endif
+
+#ifdef CONFIG_TASKS_RUDE_RCU
+ cblist_init_generic(&rcu_tasks_rude);
+#endif
+
+#ifdef CONFIG_TASKS_TRACE_RCU
+ cblist_init_generic(&rcu_tasks_trace);
+#endif
+}
+
void __init rcu_init_tasks_generic(void)
{
#ifdef CONFIG_TASKS_RCU
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index fec804b790803..705c0d16850aa 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -261,4 +261,5 @@ void __init rcu_init(void)
{
open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
rcu_early_boot_tests();
+ tasks_cblist_init_generic();
}
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 31f3a61f9c384..4f4aec64039f0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -5601,6 +5601,8 @@ void __init rcu_init(void)
(void)start_poll_synchronize_rcu_expedited();

rcu_test_sync_prims();
+
+ tasks_cblist_init_generic();
}

#include "tree_stall.h"