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

[Dennis Zhou <dennis@kernel.org>]

Hello,

On Thu, Apr 07, 2022 at 03:57:52PM -0700, Andrew Morton wrote:
> (cc Ming Lei)
> 
> On Thu,  7 Apr 2022 18:33:35 +0800 Qi Zheng <zhengqi.arch@bytedance.com> 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.
> 
> Thanks.  I'll grab this, but shall await input from others before doing
> anything else with it.
> 

Seems right to me. The percpu_ref protects the __percpu_ref_exit(), not
the waiters.

Acked-by: Dennis Zhou <dennis@kernel.org>

Thanks,
Dennis

> > Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> > +++ b/lib/percpu-refcount.c
> > @@ -154,13 +154,14 @@ static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
> >  
> >  	data->confirm_switch(ref);
> >  	data->confirm_switch = NULL;
> > -	wake_up_all(&percpu_ref_switch_waitq);
> >  
> >  	if (!data->allow_reinit)
> >  		__percpu_ref_exit(ref);
> >  
> >  	/* drop ref from percpu_ref_switch_to_atomic() */
> >  	percpu_ref_put(ref);
> > +
> > +	wake_up_all(&percpu_ref_switch_waitq);
> >  }
> >  
> >  static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
>