On 2022/4/8 1:57 PM, Dennis Zhou wrote:
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@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:
Did you find this bug through code inspection or was the finding
motivated by a production incident?
I find this bug through code inspection, because I want to use
percpu_ref_switch_to_atomic_sync()+percpu_ref_is_zero() to do something
similar.
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
Agree, I manually added the delay in wake_up_all() and percpu_ref_put()
to trigger the case B.
faster than an atomic decrement. The q_usage_counter allows reinit so it
skips the __percpu_ref_exit() call.
Thanks,
Dennis
--
Thanks,
Qi
