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35a2897c2a
Steven Rostedt reported a potential race in RCU core because of
swake_up():
CPU0 CPU1
---- ----
__call_rcu_core() {
spin_lock(rnp_root)
need_wake = __rcu_start_gp() {
rcu_start_gp_advanced() {
gp_flags = FLAG_INIT
}
}
rcu_gp_kthread() {
swait_event_interruptible(wq,
gp_flags & FLAG_INIT) {
spin_lock(q->lock)
*fetch wq->task_list here! *
list_add(wq->task_list, q->task_list)
spin_unlock(q->lock);
*fetch old value of gp_flags here *
spin_unlock(rnp_root)
rcu_gp_kthread_wake() {
swake_up(wq) {
swait_active(wq) {
list_empty(wq->task_list)
} * return false *
if (condition) * false *
schedule();
In this case, a wakeup is missed, which could cause the rcu_gp_kthread
waits for a long time.
The reason of this is that we do a lockless swait_active() check in
swake_up(). To fix this, we can either 1) add a smp_mb() in swake_up()
before swait_active() to provide the proper order or 2) simply remove
the swait_active() in swake_up().
The solution 2 not only fixes this problem but also keeps the swait and
wait API as close as possible, as wake_up() doesn't provide a full
barrier and doesn't do a lockless check of the wait queue either.
Moreover, there are users already using swait_active() to do their quick
checks for the wait queues, so it make less sense that swake_up() and
swake_up_all() do this on their own.
This patch then removes the lockless swait_active() check in swake_up()
and swake_up_all().
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170615041828.zk3a3sfyudm5p6nl@tardis
Signed-off-by: Ingo Molnar <mingo@kernel.org>
118 lines
2.9 KiB
C
118 lines
2.9 KiB
C
#include <linux/sched/signal.h>
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#include <linux/swait.h>
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void __init_swait_queue_head(struct swait_queue_head *q, const char *name,
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struct lock_class_key *key)
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{
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raw_spin_lock_init(&q->lock);
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lockdep_set_class_and_name(&q->lock, key, name);
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INIT_LIST_HEAD(&q->task_list);
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}
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EXPORT_SYMBOL(__init_swait_queue_head);
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/*
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* The thing about the wake_up_state() return value; I think we can ignore it.
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*
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* If for some reason it would return 0, that means the previously waiting
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* task is already running, so it will observe condition true (or has already).
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*/
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void swake_up_locked(struct swait_queue_head *q)
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{
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struct swait_queue *curr;
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if (list_empty(&q->task_list))
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return;
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curr = list_first_entry(&q->task_list, typeof(*curr), task_list);
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wake_up_process(curr->task);
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list_del_init(&curr->task_list);
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}
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EXPORT_SYMBOL(swake_up_locked);
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void swake_up(struct swait_queue_head *q)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&q->lock, flags);
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swake_up_locked(q);
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raw_spin_unlock_irqrestore(&q->lock, flags);
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}
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EXPORT_SYMBOL(swake_up);
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/*
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* Does not allow usage from IRQ disabled, since we must be able to
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* release IRQs to guarantee bounded hold time.
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*/
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void swake_up_all(struct swait_queue_head *q)
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{
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struct swait_queue *curr;
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LIST_HEAD(tmp);
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raw_spin_lock_irq(&q->lock);
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list_splice_init(&q->task_list, &tmp);
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while (!list_empty(&tmp)) {
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curr = list_first_entry(&tmp, typeof(*curr), task_list);
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wake_up_state(curr->task, TASK_NORMAL);
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list_del_init(&curr->task_list);
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if (list_empty(&tmp))
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break;
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raw_spin_unlock_irq(&q->lock);
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raw_spin_lock_irq(&q->lock);
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}
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raw_spin_unlock_irq(&q->lock);
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}
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EXPORT_SYMBOL(swake_up_all);
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void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait)
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{
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wait->task = current;
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if (list_empty(&wait->task_list))
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list_add(&wait->task_list, &q->task_list);
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}
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void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&q->lock, flags);
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__prepare_to_swait(q, wait);
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set_current_state(state);
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raw_spin_unlock_irqrestore(&q->lock, flags);
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}
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EXPORT_SYMBOL(prepare_to_swait);
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long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state)
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{
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if (signal_pending_state(state, current))
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return -ERESTARTSYS;
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prepare_to_swait(q, wait, state);
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return 0;
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}
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EXPORT_SYMBOL(prepare_to_swait_event);
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void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
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{
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__set_current_state(TASK_RUNNING);
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if (!list_empty(&wait->task_list))
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list_del_init(&wait->task_list);
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}
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void finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
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{
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unsigned long flags;
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__set_current_state(TASK_RUNNING);
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if (!list_empty_careful(&wait->task_list)) {
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raw_spin_lock_irqsave(&q->lock, flags);
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list_del_init(&wait->task_list);
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raw_spin_unlock_irqrestore(&q->lock, flags);
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}
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}
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EXPORT_SYMBOL(finish_swait);
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