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

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Date:   Thu, 7 Apr 2022 20:54:19 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     Muchun Song <songmuchun@...edance.com>, dennis@...nel.org,
        tj@...nel.org, cl@...ux.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, zhouchengming@...edance.com
Subject: Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put()
 completes

On Fri, 8 Apr 2022 11:50:05 +0800 Qi Zheng <zhengqi.arch@...edance.com> wrote:

> 
> 
> On 2022/4/8 10:54 AM, Muchun Song wrote:
> > 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@...edance.com>
> > 
> > Are any users affected by this?  If so, I think a Fixes tag
> > is necessary.
> 
> Looks all current users(blk_pre_runtime_suspend() and set_in_sync()) are
> affected by this.
> 
> I see that this patch has been merged into the mm tree, can Andrew help
> me add the following Fixes tag?

Andrew is helpful ;)

Do you see reasons why we should backport this into -stable trees? 
It's 8 years old, so my uninformed guess is "no"?