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[PATCH v2 1/1] percpu-refcount: do not forget to rcu_barrier() just before freeing

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percpu issues some RCU callbacks to synchronize its state, so before
freeing we have to wait all those callbacks to finish.

E.g. the following simple sequence on stack causes nasty crash:

    struct percpu_ref ref;

    percpu_ref_init(&ref, release, 0, GFP_KERNEL);
    percpu_ref_kill(&ref);
    percpu_ref_exit(&ref);

Also this patch includes inition to NULL of confirm_switch callback.
Without this inition you have to zero out a chunk of memory or kernel
frightfully complains with WARN_ON_ONCE(ref->confirm_switch) at
__percpu_ref_switch_to_atomic.

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: linux-kernel@vger.kernel.org
---
 v2:
   - just sending one more time because v1 was accidently
     sent as a reply to not related issue.

 lib/percpu-refcount.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 6111bcb..ddf934b 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -82,6 +82,7 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
 	atomic_long_set(&ref->count, start_count);
 
 	ref->release = release;
+	ref->confirm_switch = NULL;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(percpu_ref_init);
@@ -101,6 +102,7 @@ void percpu_ref_exit(struct percpu_ref *ref)
 	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
 
 	if (percpu_count) {
+		rcu_barrier_sched();
 		free_percpu(percpu_count);
 		ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
 	}
-- 
2.9.0

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[PATCH v2 1/1] percpu-refcount: do not forget to rcu_barrier() just before freeing Roman Pen <roman.penyaev@profitbricks.com> - 2016-08-10 22:00 +0200
  Re: [PATCH v2 1/1] percpu-refcount: do not forget to rcu_barrier()  just before freeing Tejun Heo <tj@kernel.org> - 2016-08-11 00:10 +0200
    Re: [PATCH v2 1/1] percpu-refcount: do not forget to rcu_barrier()  just before freeing Roman Penyaev <roman.penyaev@profitbricks.com> - 2016-08-11 11:10 +0200
      Re: [PATCH v2 1/1] percpu-refcount: do not forget to rcu_barrier()  just before freeing Tejun Heo <tj@kernel.org> - 2016-08-11 18:30 +0200
        Re: [PATCH v2 1/1] percpu-refcount: do not forget to rcu_barrier()  just before freeing Roman Penyaev <roman.penyaev@profitbricks.com> - 2016-08-11 19:40 +0200

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