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SubjectRe: INFO: task hung in blk_queue_enter
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I checked counter values using debug printk() patch shown below, and
found that q->q_usage_counter.count == 1 when this deadlock occurs.

Since sum of percpu_count did not change after percpu_ref_kill(), this is
not a race condition while folding percpu counter values into atomic counter
value. That is, for some reason, someone who is responsible for calling
percpu_ref_put(&q->q_usage_counter) (presumably via blk_queue_exit()) is
unable to call percpu_ref_put().

diff --git a/block/blk-core.c b/block/blk-core.c
index 85909b4..6933020 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -908,6 +908,12 @@ struct request_queue *blk_alloc_queue(gfp_t gfp_mask)
}
EXPORT_SYMBOL(blk_alloc_queue);

+static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
+{
+ return (unsigned long __percpu *)
+ (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
+}
+
/**
* blk_queue_enter() - try to increase q->q_usage_counter
* @q: request queue pointer
@@ -950,10 +956,22 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
*/
smp_rmb();

- wait_event(q->mq_freeze_wq,
- (atomic_read(&q->mq_freeze_depth) == 0 &&
- (preempt || !blk_queue_preempt_only(q))) ||
- blk_queue_dying(q));
+ while (wait_event_timeout(q->mq_freeze_wq,
+ (atomic_read(&q->mq_freeze_depth) == 0 &&
+ (preempt || !blk_queue_preempt_only(q))) ||
+ blk_queue_dying(q), 3 * HZ) == 0) {
+ struct percpu_ref *ref = &q->q_usage_counter;
+ unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
+ unsigned long count = 0;
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ count += *per_cpu_ptr(percpu_count, cpu);
+
+ printk("%s(%d): %px %ld %ld\n", current->comm, current->pid,
+ ref, atomic_long_read(&ref->count), count);
+ }
+
if (blk_queue_dying(q))
return -ENODEV;
}
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 9f96fa7..72773ce 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -133,8 +133,8 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
for_each_possible_cpu(cpu)
count += *per_cpu_ptr(percpu_count, cpu);

- pr_debug("global %ld percpu %ld",
- atomic_long_read(&ref->count), (long)count);
+ printk("%px global %ld percpu %ld\n", ref,
+ atomic_long_read(&ref->count), (long)count);

/*
* It's crucial that we sum the percpu counters _before_ adding the sum
@@ -150,6 +150,8 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
*/
atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);

+ printk("%px global %ld\n", ref, atomic_long_read(&ref->count));
+
WARN_ONCE(atomic_long_read(&ref->count) <= 0,
"percpu ref (%pf) <= 0 (%ld) after switching to atomic",
ref->release, atomic_long_read(&ref->count));
If I change blk_queue_enter() not to wait at wait_event() if
q->mq_freeze_depth != 0, this deadlock problem does not occur.

Also, I found that if blk_freeze_queue_start() tries to wait for
counters to become 1 before calling percpu_ref_kill() (like shown
below), this deadlock problem does not occur.

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9ce9cac..4bff534 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -134,12 +134,36 @@ void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part,
blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight_rw, &mi);
}

+#define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))
+
+static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
+{
+ return (unsigned long __percpu *)
+ (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
+}
+
void blk_freeze_queue_start(struct request_queue *q)
{
int freeze_depth;

freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
if (freeze_depth == 1) {
+ int i;
+ for (i = 0; i < 10; i++) {
+ struct percpu_ref *ref = &q->q_usage_counter;
+ unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
+ unsigned long count = 0;
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ count += *per_cpu_ptr(percpu_count, cpu);
+
+ if (atomic_long_read(&ref->count) + count - PERCPU_COUNT_BIAS == 1)
+ break;
+ printk("%s(%d):! %px %ld %ld\n", current->comm, current->pid,
+ ref, atomic_long_read(&ref->count), count);
+ schedule_timeout_uninterruptible(HZ / 10);
+ }
percpu_ref_kill(&q->q_usage_counter);
if (q->mq_ops)
blk_mq_run_hw_queues(q, false);
But I don't know how to find someone who is failing to call percpu_ref_put()...

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