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

[Dennis Zhou <dennis@kernel.org>]

On Fri, Apr 08, 2022 at 12:14:54PM +0800, Qi Zheng wrote:
> 
> 
> On 2022/4/8 12:10 PM, Andrew Morton wrote:
> > On Fri, 8 Apr 2022 12:06:20 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> > 
> > > 
> > > 
> > > > > > > Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.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?
> 
> 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:
> 

Did you find this bug through code inspection or was the finding
motivated by a production incident?

The usage in block/blk-pm.c looks problematic, but I'm guessing this is
a really, really hard bug to trigger. You need to have the wake up be
faster than an atomic decrement. The q_usage_counter allows reinit so it
skips the __percpu_ref_exit() call.

Thanks,
Dennis