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

[Qi Zheng]

On 2022/4/8 12:10 PM, Andrew Morton wrote:
> On Fri, 8 Apr 2022 12:06:20 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
>
> > > > > > 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"?
> >
> > 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?

The impact on the current user is that it is possible to miss an 
opportunity to reach 0 due to the case B in the commit message:

/* The value of &ref is unstable! */
percpu_ref_is_zero(&ref)
						(B)percpu_ref_put(ref);

Thanks,
Qi

>

--
Thanks,
Qi