Re: [PATCH 3.12 49/82] xen/gntdevt: Fix race condition in gntdev_release()

[Jiri Slaby]

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On 08/25/2015, 01:52 PM, Marek Marczykowski-GÃrecki wrote:
>>> --- a/drivers/xen/gntdev.c +++ b/drivers/xen/gntdev.c @@
>>> -529,12 +529,14 @@ static int gntdev_release(struct inode
>>> *inode, struct file *flip)
>>> 
>>> pr_debug("priv %p\n", priv);
>>> 
>>> +	mutex_lock(&priv->lock);
>> 
>> Since 3.12 doesn't seem to include 1401c00e59ea ("xen/gntdev:
>> convert priv->lock to a mutex"), this shouldn't be applied as
>> priv->lock is actually a spinlock.  So, you'll need to pick
>> 1401c00e59ea or backport this patch using the appropriate locking
>> directives.  Not sure what's the best solution.  Maybe Marek or
>> David can help...?
> 
> I've used spinlock approach for some time (on 3.18.x) and it works
> ok. This applies also to 3.10 and 3.14 of course.
> 
> Patch here: 
> https://raw.githubusercontent.com/QubesOS/qubes-linux-kernel/stable-3.
18/patches.xen/0001-xen-grant-fix-race-condition-in-gntdev_release.patch
>
>  and here:
> 
> From b876e14888bdafa112c3265e6420543fa74aa709 Mon Sep 17 00:00:00
> 2001 From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?= 
> <marmarek@xxxxxxxxxxxxxxxxxxxxxx> Date: Fri, 26 Jun 2015 02:16:49
> +0200 Subject: [PATCH] xen/grant: fix race condition in
> gntdev_release MIME-Version: 1.0 Content-Type: text/plain;
> charset=UTF-8 Content-Transfer-Encoding: 8bit Organization:
> Invisible Things Lab Cc: Marek Marczykowski-GÃrecki
> <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> 
> While gntdev_release is called, MMU notifier is still registered
> and will traverse priv->maps list even if no pages are mapped
> (which is the case - gntdev_release is called after all). But
> gntdev_release will clear that list, so make sure that only one of
> those things happens at the same time.
> 
> Signed-off-by: Marek Marczykowski-GÃrecki
> <marmarek@xxxxxxxxxxxxxxxxxxxxxx> --- drivers/xen/gntdev.c | 2 ++ 1
> file changed, 2 insertions(+)
> 
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c index
> 8927485..4bd23bb 100644 --- a/drivers/xen/gntdev.c +++
> b/drivers/xen/gntdev.c @@ -568,12 +568,14 @@ static int
> gntdev_release(struct inode *inode, struct file *flip)
> 
> pr_debug("priv %p\n", priv);
> 
> +	spin_lock(&priv->lock); while (!list_empty(&priv->maps)) { map =
> list_entry(priv->maps.next, struct grant_map, next); 
> list_del(&map->next); gntdev_put_map(NULL /* already removed */,
> map); } WARN_ON(!list_empty(&priv->freeable_maps)); +
> spin_unlock(&priv->lock);

Hmm, but e.g.
gntdev_put_map
 -> gntdev_free_map
   -> free_xenballooned_pages
     -> mutex_lock

means sleep inside atomic, right?

thanks,
- -- 
js
suse labs
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