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

From: Qi Zheng
Date: Fri Apr 08 2022 - 00:15:07 EST




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