linux-kernel - [PATCH] percpu_ref: call wake_up_all() after percpu_ref

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Date:   Thu,  7 Apr 2022 18:33:35 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     dennis@...nel.org, tj@...nel.org, cl@...ux.com,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        zhouchengming@...edance.com, songmuchun@...edance.com,
        Qi Zheng <zhengqi.arch@...edance.com>
Subject: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put() completes

In the percpu_ref_call_confirm_rcu(), we call the wake_up_all()
before calling percpu_ref_put(), which will cause the value of
percpu_ref to be unstable when percpu_ref_switch_to_atomic_sync()
returns.

	CPU0				CPU1

percpu_ref_switch_to_atomic_sync(&ref)
--> percpu_ref_switch_to_atomic(&ref)
    --> percpu_ref_get(ref);	/* put after confirmation */
	call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);

					percpu_ref_switch_to_atomic_rcu
					--> percpu_ref_call_confirm_rcu
					    --> data->confirm_switch = NULL;
						wake_up_all(&percpu_ref_switch_waitq);

    /* here waiting to wake up */
    wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
						(A)percpu_ref_put(ref);
/* The value of &ref is unstable! */
percpu_ref_is_zero(&ref)
						(B)percpu_ref_put(ref);

As shown above, assuming that the counts on each cpu add up to 0 before
calling percpu_ref_switch_to_atomic_sync(), we expect that after switching
to atomic mode, percpu_ref_is_zero() can return true. But actually it will
return different values in the two cases of A and B, which is not what
we expected.

Maybe the original purpose of percpu_ref_switch_to_atomic_sync() is
just to ensure that the conversion to atomic mode is completed, but it
should not return with an extra reference count.

Calling wake_up_all() after percpu_ref_put() ensures that the value of
percpu_ref is stable after percpu_ref_switch_to_atomic_sync() returns.
So just do it.

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 lib/percpu-refcount.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index af9302141bcf..b11b4152c8cd 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -154,13 +154,14 @@ static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)

 	data->confirm_switch(ref);
 	data->confirm_switch = NULL;
-	wake_up_all(&percpu_ref_switch_waitq);

 	if (!data->allow_reinit)
 		__percpu_ref_exit(ref);

 	/* drop ref from percpu_ref_switch_to_atomic() */
 	percpu_ref_put(ref);
+
+	wake_up_all(&percpu_ref_switch_waitq);
 }

 static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
-- 
2.20.1