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

[Qi Zheng]

On 2022/4/8 11:54 AM, Andrew Morton wrote:
> On Fri, 8 Apr 2022 11:50:05 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
>
> > On 2022/4/8 10:54 AM, Muchun Song wrote:
> > > On Thu, Apr 07, 2022 at 06:33:35PM +0800, Qi Zheng wrote:
> > > > In the percpu_ref_call_confirm_rcu(), we call the wake_up_all()
> > > > before calling percpu_ref_put(), which will cause the value of
> > > > percpu_ref to be unstable when percpu_ref_switch_to_atomic_sync()
> > > > returns.
> > > >
> > > > 	CPU0				CPU1
> > > >
> > > > percpu_ref_switch_to_atomic_sync(&ref)
> > > > --> percpu_ref_switch_to_atomic(&ref)
> > > >       --> percpu_ref_get(ref);	/* put after confirmation */
> > > > 	call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);
> > > >
> > > > 					percpu_ref_switch_to_atomic_rcu
> > > > 					--> percpu_ref_call_confirm_rcu
> > > > 					    --> data->confirm_switch = NULL;
> > > > 						wake_up_all(&percpu_ref_switch_waitq);
> > > >
> > > >       /* here waiting to wake up */
> > > >       wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
> > > > 						(A)percpu_ref_put(ref);
> > > > /* The value of &ref is unstable! */
> > > > percpu_ref_is_zero(&ref)
> > > > 						(B)percpu_ref_put(ref);
> > > >
> > > > As shown above, assuming that the counts on each cpu add up to 0 before
> > > > calling percpu_ref_switch_to_atomic_sync(), we expect that after switching
> > > > to atomic mode, percpu_ref_is_zero() can return true. But actually it will
> > > > return different values in the two cases of A and B, which is not what
> > > > we expected.
> > > >
> > > > Maybe the original purpose of percpu_ref_switch_to_atomic_sync() is
> > > > just to ensure that the conversion to atomic mode is completed, but it
> > > > should not return with an extra reference count.
> > > >
> > > > Calling wake_up_all() after percpu_ref_put() ensures that the value of
> > > > percpu_ref is stable after percpu_ref_switch_to_atomic_sync() returns.
> > > > So just do it.
> > > >
> > > > 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.

>

--
Thanks,
Qi