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Date:   Fri, 8 Apr 2022 10:54:57 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     dennis@kernel.org, tj@kernel.org, cl@linux.com,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, zhouchengming@bytedance.com
Subject: Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put()
 completes
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References: <20220407103335.36885-1-zhengqi.arch@bytedance.com>
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On Thu, Apr 07, 2022 at 06:33:35PM +0800, Qi Zheng wrote:
> 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@bytedance.com>

Are any users affected by this?  If so, I think a Fixes tag
is necessary.

The fix LGTM.

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks.