Re: [PATCH 1/6] xen-gntdev: Fix circular locking dependency

[Daniel De Graaf <dgdegra@tycho.nsa.gov>]

On 12/14/2010 04:11 PM, Jeremy Fitzhardinge wrote:
> On 12/14/2010 06:55 AM, Daniel De Graaf wrote:
>> apply_to_page_range will acquire PTE lock while priv->lock is held, and
>> mn_invl_range_start tries to acquire priv->lock with PTE already held.
>> Fix by not holding priv->lock during the entire map operation.
> 
> Is priv->lock needed to protect the contents of map?
> 
>     J

No, since the map can only be mapped once (checked by map->vma assignment
while the lock is held). The unmap ioctl will return -EBUSY until
an munmap() is called on the area, so it also can't race, and the other
users are only active once the mmap operation completes.

> 
>> Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
>> ---
>>  drivers/xen/gntdev.c |   19 +++++++++----------
>>  1 files changed, 9 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
>> index a33e443..c582804 100644
>> --- a/drivers/xen/gntdev.c
>> +++ b/drivers/xen/gntdev.c
>> @@ -581,23 +581,22 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
>>  	if (!(vma->vm_flags & VM_WRITE))
>>  		map->flags |= GNTMAP_readonly;
>>  
>> +	spin_unlock(&priv->lock);
>> +
>>  	err = apply_to_page_range(vma->vm_mm, vma->vm_start,
>>  				  vma->vm_end - vma->vm_start,
>>  				  find_grant_ptes, map);
>> -	if (err) {
>> -		goto unlock_out;
>> -		if (debug)
>> -			printk("%s: find_grant_ptes() failure.\n", __FUNCTION__);
>> -	}
>> +	if (err)
>> +		return err;
>>  
>>  	err = map_grant_pages(map);
>> -	if (err) {
>> -		goto unlock_out;
>> -		if (debug)
>> -			printk("%s: map_grant_pages() failure.\n", __FUNCTION__);
>> -	}
>> +	if (err)
>> +		return err;
>> +
>>  	map->is_mapped = 1;
>>  
>> +	return 0;
>> +
>>  unlock_out:
>>  	spin_unlock(&priv->lock);
>>  	return err;
>