Re: [PATCH] padata: Use RCU when fetching pd from do_serial

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 16, 2019 at 08:57:04PM +0800, Herbert Xu wrote:
> On Tue, Jul 16, 2019 at 01:14:10PM +0200, Steffen Klassert wrote:
> >
> > Maybe we can fix it if we call padata_free_pd() from
> > padata_serial_worker() when it sent out the last object.
> 
> How about using RCU?
> 
> We still need to fix up the refcnt if it's supposed to limit the
> overall number of outstanding requests.
> 
> ---8<---
> The function padata_do_serial uses parallel_data without obeying
> the RCU rules around its life-cycle.  This means that a concurrent
> padata_replace call can result in a crash.
> 
> This patch fixes it by using RCU just as we do in padata_do_parallel.
> 
> Fixes: 16295bec6398 ("padata: Generic parallelization/...")
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

> diff --git a/kernel/padata.c b/kernel/padata.c
> index 2d2fddbb7a4c..fb5dd1210d2b 100644
> --- a/kernel/padata.c
> +++ b/kernel/padata.c
> @@ -128,7 +128,7 @@ int padata_do_parallel(struct padata_instance *pinst,
>  
>  	err = 0;
>  	atomic_inc(&pd->refcnt);
> -	padata->pd = pd;
> +	padata->inst = pinst;
>  	padata->cb_cpu = cb_cpu;
>  
>  	target_cpu = padata_cpu_hash(pd);
> @@ -367,7 +368,7 @@ void padata_do_serial(struct padata_priv *padata)
>  	struct parallel_data *pd;
>  	int reorder_via_wq = 0;
>  
> -	pd = padata->pd;
> +	pd = rcu_dereference_bh(padata->inst->pd);
>  
>  	cpu = get_cpu();
>  

That's weird for not having a matching assign and lacking comments to
explain that.