Re: [PATCH] xen/scsiback: don't call scsiback_free_translation_entry() under lock

[Juergen Gross <jgross@xxxxxxxx>]

On 28.03.23 17:47, Oleksandr Tyshchenko wrote:
> On 28.03.23 11:46, Juergen Gross wrote:
>
> Hello Juergen
>
> > scsiback_free_translation_entry() shouldn't be called under spinlock,
> > as it can sleep.
> >
> > This requires to split removing a translation entry from the v2p list
> > from actually calling kref_put() for the entry.
> >
> > Reported-by: Dan Carpenter <error27@xxxxxxxxx>
> > Link: https://urldefense.com/v3/__https://lore.kernel.org/lkml/Y*JUIl64UDmdkboh@kadam/__;Kw!!GF_29dbcQIUBPA!23IKdVhamoFq8ptUnprd_TubDMObj-0QAalsGiffBHCeEdOuwrq7z4ohg92Sj0olgl0nh73oXvSr-i1zqXhY$ [lore[.]kernel[.]org]
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >    drivers/xen/xen-scsiback.c | 27 ++++++++++++++-------------
> >    1 file changed, 14 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/xen/xen-scsiback.c b/drivers/xen/xen-scsiback.c
> > index 954188b0b858..294f29cdc7aa 100644
> > --- a/drivers/xen/xen-scsiback.c
> > +++ b/drivers/xen/xen-scsiback.c
> > @@ -1010,12 +1010,6 @@ static int scsiback_add_translation_entry(struct vscsibk_info *info,
> >    	return err;
> >    }
> >    
> > -static void __scsiback_del_translation_entry(struct v2p_entry *entry)
> > -{
> > -	list_del(&entry->l);
> > -	kref_put(&entry->kref, scsiback_free_translation_entry);
> > -}
> > -
> >    /*
> >      Delete the translation entry specified
> >    */
> > @@ -1024,18 +1018,20 @@ static int scsiback_del_translation_entry(struct vscsibk_info *info,
> >    {
> >    	struct v2p_entry *entry;
> >    	unsigned long flags;
> > -	int ret = 0;
> >    
> >    	spin_lock_irqsave(&info->v2p_lock, flags);
> >    	/* Find out the translation entry specified */
> >    	entry = scsiback_chk_translation_entry(info, v);
> >    	if (entry)
> > -		__scsiback_del_translation_entry(entry);
> > -	else
> > -		ret = -ENOENT;
> > +		list_del(&entry->l);
> >    
> >    	spin_unlock_irqrestore(&info->v2p_lock, flags);
> > -	return ret;
> > +
> > +	if (!entry)
> > +		return -ENOENT;
> > +
> > +	kref_put(&entry->kref, scsiback_free_translation_entry);
> > +	return 0;
> >    }
> >    
> >    static void scsiback_do_add_lun(struct vscsibk_info *info, const char *state,
> > @@ -1239,14 +1235,19 @@ static void scsiback_release_translation_entry(struct vscsibk_info *info)
> >    {
> >    	struct v2p_entry *entry, *tmp;
> >    	struct list_head *head = &(info->v2p_entry_lists);
> > +	struct list_head tmp_list;
>
>
> I would use LIST_HEAD(tmp_list);

There is no need to initialize it, so I think I will keep it as is.

> >    	unsigned long flags;
> >    
> >    	spin_lock_irqsave(&info->v2p_lock, flags);
> >    
> > -	list_for_each_entry_safe(entry, tmp, head, l)
> > -		__scsiback_del_translation_entry(entry);
> > +	list_cut_before(&tmp_list, head, head);
>
> so we just move all entries from head to tmp_list here to be processed...

Correct.

> >    
> >    	spin_unlock_irqrestore(&info->v2p_lock, flags);
>
> ... when the lock is not held, ok
>
> Patch LGTM, but one (maybe stupid) question to clarify.
>
> Why do we need to use a lock here in the first place? The
> scsiback_release_translation_entry() gets called when the driver
> instance is about to be removed and *after* the disconnection from
> otherend (so no requests are expected), so what else might cause this
> list to be accessed concurrently?

Maybe nothing, but I think it is good practice to keep the lock in order
to avoid future code changes to cause problems.


Juergen

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