[PATCH] percpu-refcount: Add a WARN() for ref going negative

[Kent Overstreet]

AIO had a missing get, which led to an ioctx leak - after percpu_ref_kill() the
ref was 0 so percpu_ref_put() never saw it hit 0.

This wasn't noticed at the time because it all happened completely silently,
this adds a WARN() which would've caught the aio bug.

Signed-off-by: Kent Overstreet <kmo@xxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
---
 lib/percpu-refcount.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 1a53d497a8..32538f12f8 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -120,6 +120,9 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu)
 
 	atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
 
+	WARN(atomic_read(&ref->count) <= 0,
+	     "percpu ref <= 0 (%i)", atomic_read(&ref->count));
+
 	/* @ref is viewed as dead on all CPUs, send out kill confirmation */
 	if (ref->confirm_kill)
 		ref->confirm_kill(ref);
-- 
1.8.5.2

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