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

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

[Juergen Gross]

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@gmail.com>
Link: https://lore.kernel.org/lkml/Y+JUIl64UDmdkboh@kadam/
Signed-off-by: Juergen Gross <jgross@suse.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;
 	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);
 
 	spin_unlock_irqrestore(&info->v2p_lock, flags);
+
+	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)
-- 
2.35.3

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

[Oleksandr Tyshchenko]

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@gmail.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@suse.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)

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

[Juergen Gross]

[-- Attachment #1.1.1: Type: text/plain, Size: 3515 bytes --]

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@gmail.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@suse.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


[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3149 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]

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

[Oleksandr Tyshchenko]

On 29.03.23 09:20, Juergen Gross wrote:

Hello Juergen


> 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@gmail.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@suse.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.


Thanks for the explanation, it sounds reasonable to me.

Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

> 
> 
> Juergen
>