Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put() completes

From: Andrew Morton
Date: Thu Apr 07 2022 - 23:54:30 EST


On Fri, 8 Apr 2022 11:50:05 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> >
> > 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"?