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

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Date:   Wed, 29 Mar 2023 08:20:57 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.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 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@...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);

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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