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

[Qi Zheng]

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?

Fixes: 490c79a65708 ("percpu_ref: decouple switching to atomic mode and 
killing")

Thanks,
Qi

> The fix LGTM.
>
> Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>
> Thanks.

--
Thanks,
Qi