linux-kernel - Re: [PATCH] xen/scsiback: don't call scsiback_free_translation

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Message-ID: <df07e1e3-ebe0-65c6-58c0-831739b9ed4c@epam.com>
Date:   Tue, 28 Mar 2023 15:47:05 +0000
From:   Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>
To:     Juergen Gross <jgross@...e.com>
CC:     Stefano Stabellini <sstabellini@...nel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Dan Carpenter <error27@...il.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen/scsiback: don't call
 scsiback_free_translation_entry() under lock



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@...il.com>
> 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@...e.com>
> ---
>   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);

>   	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...

>   
>   	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?


> +
> +	list_for_each_entry_safe(entry, tmp, &tmp_list, l) {
> +		list_del(&entry->l);
> +		kref_put(&entry->kref, scsiback_free_translation_entry);
> +	}
>   }
>   
>   static void scsiback_remove(struct xenbus_device *dev)