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

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Message-Id: <20220407211018.875696691e4411a7b5c8f63f@linux-foundation.org>
Date:   Thu, 7 Apr 2022 21:10:18 -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 12:06:20 +0800 Qi Zheng <zhengqi.arch@...edance.com> wrote:

> 
> 
> >>>> 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"?
> 
> Hmm, although the commit 490c79a65708 add wake_up_all(), it is no
> problem for the usage at that time, maybe the correct Fixes tag is the
> following:
> 
> Fixes: 210f7cdcf088 ("percpu-refcount: support synchronous switch to 
> atomic mode.")
> 
> But in fact, there is no problem with it, but all current users expect
> the refcount is stable after percpu_ref_switch_to_atomic_sync() returns.
> 
> I have no idea as which Fixes tag to add.

Well the solution to that problem is to add cc:stable and let Greg
figure it out ;)

The more serious question is "should we backport this".  What is the
end-user-visible impact of the bug?  Do our users need the fix or not?