linux-kernel - Re: [PATCH 09/23] ns: Introduce ns_idr to be able to iterate all allocated namespaces in the system

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Date:   Thu, 30 Jul 2020 15:15:37 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     viro@...iv.linux.org.uk, adobriyan@...il.com, davem@...emloft.net,
        ebiederm@...ssion.com, akpm@...ux-foundation.org,
        christian.brauner@...ntu.com, areber@...hat.com, serge@...lyn.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 09/23] ns: Introduce ns_idr to be able to iterate all
 allocated namespaces in the system

On Thu, Jul 30, 2020 at 05:12:09PM +0300, Kirill Tkhai wrote:
> On 30.07.2020 16:56, Matthew Wilcox wrote:
> > On Thu, Jul 30, 2020 at 04:32:22PM +0300, Kirill Tkhai wrote:
> >> On 30.07.2020 15:23, Matthew Wilcox wrote:
> >>> xa_erase_irqsave();
> >>
> >> static inline void *xa_erase_irqsave(struct xarray *xa, unsigned long index)
> >> {
> >> 	unsigned long flags;
> >>         void *entry;
> >>
> >>         xa_lock_irqsave(xa, flags);
> >>         entry = __xa_erase(xa, index);
> >>         xa_unlock_irqrestore(xa, flags);
> >>
> >>         return entry;
> >> }
> > 
> > was there a question here?
> 
> No, I just I will add this in separate patch.

Ah, yes.  Thanks!

> >>>> +struct ns_common *ns_get_next(unsigned int *id)
> >>>> +{
> >>>> +	struct ns_common *ns;
> >>>> +
> >>>> +	if (*id < PROC_NS_MIN_INO - 1)
> >>>> +		*id = PROC_NS_MIN_INO - 1;
> >>>> +
> >>>> +	*id += 1;
> >>>> +	*id -= PROC_NS_MIN_INO;
> >>>> +
> >>>> +	rcu_read_lock();
> >>>> +	do {
> >>>> +		ns = idr_get_next(&ns_idr, id);
> >>>> +		if (!ns)
> >>>> +			break;
> >>>
> >>> xa_find_after();
> >>>
> >>> You'll want a temporary unsigned long to work with ...
> >>>
> >>>> +		if (!refcount_inc_not_zero(&ns->count)) {
> >>>> +			ns = NULL;
> >>>> +			*id += 1;
> >>>
> >>> you won't need this increment.
> >>
> >> Why? I don't see a way xarray allows to avoid this.
> > 
> > It's embedded in xa_find_after().
>  
> How is it embedded to check ns->count that it knows nothing?

I meant you won't need to increment '*id'.  The refcount is, of course,
your business.