Re: [PATCH] fs/resctrl: Fix deadlock for errors during mount

["Luck, Tony" <tony.luck@intel.com>]

On Fri, May 01, 2026 at 04:17:18PM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> On 5/1/26 11:56 AM, Tony Luck wrote:
> > Sashiko noticed[1] a deadlock in the resctrl mount code.
> > 
> > rdt_get_tree() acquires rdtgroup_mutex before calling kernfs_get_tree(). If
> > superblock setup fails inside kernfs_get_tree(), the VFS calls kill_sb on
> > the same thread before the call returns.  rdt_kill_sb() unconditionally
> > attempts to acquire rdtgroup_mutex and deadlock occurs.
> 
> Thank you for addressing this.
> 
> > 
> > Add a boolean rdt_kill_sb_locked flag. Set it for the duration of
> > kernfs_get_tree() and check in rdt_kill_sb() to determine if locks
> > are already held.
> > 
> 
> ...
> 
> > diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> > index 5dfdaa6f9d8f..8544020ef420 100644
> > --- a/fs/resctrl/rdtgroup.c
> > +++ b/fs/resctrl/rdtgroup.c
> > @@ -2782,6 +2782,9 @@ static void schemata_list_destroy(void)
> >  	}
> >  }
> >  
> > +/* Protected by the serialized mount path (rdtgroup_mutex + resctrl_mounted). */
> 
> I interpret above to mean that every access to rdt_kill_sb_locked can be expected to
> be done with rdtgroup_mutex held ...

The comment could be much more descriptive about locking and limited use
case.

> > +static bool rdt_kill_sb_locked;
> > +
> >  static int rdt_get_tree(struct fs_context *fc)
> >  {
> >  	struct rdt_fs_context *ctx = rdt_fc2context(fc);
> > @@ -2855,7 +2858,9 @@ static int rdt_get_tree(struct fs_context *fc)
> >  	if (ret)
> >  		goto out_mondata;
> >  
> > +	rdt_kill_sb_locked = true;
> >  	ret = kernfs_get_tree(fc);
> > +	rdt_kill_sb_locked = false;
> >  	if (ret < 0)
> >  		goto out_psl;
> >  
> > @@ -3173,8 +3178,10 @@ static void rdt_kill_sb(struct super_block *sb)
> >  {
> >  	struct rdt_resource *r;
> >  
> > -	cpus_read_lock();
> > -	mutex_lock(&rdtgroup_mutex);
> > +	if (!rdt_kill_sb_locked) {
> > +		cpus_read_lock();
> > +		mutex_lock(&rdtgroup_mutex);
> 
> ... but here clearly rdt_kill_sb_locked can be accessed without rdtgroup_mutex held.

A much better name for this flag would be "resctrl_mount_in_progress". With
The header comment noting that it is set-and cleared inside
rdtgroup_mutex protected code, it is used only in rdt_kill_sb().
This specific use case seems safe as there are only call chains leading
to rdt_kill_sb():
	1) Error cleanup from failure of kernfs_fill_super() within the
	   call to kernfs_get_tree() [rdtgroup_mutex still held in this
	   case]
	2) From user call to unmount the filesystem. In which case
	   rdt_get_tree() must have completed successfully. Any new
	   calls are blocked from changing this flag by the early exit
	   based on resctrl_mounted.
> 
> It appears that while this change claims that rdt_kill_sb_locked is protected the
> implementation instead seems to actually be "this works for the scenarios cared
> about here" which I understand to be based on considerations of how the filesystem
> code interacts with resctrl callbacks _today_.
> 
> > +	}
> >  
> >  	rdt_disable_ctx();
> >  
> > @@ -3189,8 +3196,10 @@ static void rdt_kill_sb(struct super_block *sb)
> >  		resctrl_arch_disable_mon();
> >  	resctrl_mounted = false;
> >  	kernfs_kill_sb(sb);
> > -	mutex_unlock(&rdtgroup_mutex);
> > -	cpus_read_unlock();
> > +	if (!rdt_kill_sb_locked) {
> > +		mutex_unlock(&rdtgroup_mutex);
> > +		cpus_read_unlock();
> > +	}
> >  }
> >  
> >  static struct file_system_type rdt_fs_type = {
> 
> Did you or your AI assistant consider running kernfs_get_tree() without rdtgroup_mutex
> and CPU hotplug lock held? Consider, for example:

Not considered. Thanks for the suggestion ... But, see below.

> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index 36d21652616e..9ee6295d6521 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
> @@ -2892,10 +2892,6 @@ static int rdt_get_tree(struct fs_context *fc)
>  	if (ret)
>  		goto out_mondata;
>  
> -	ret = kernfs_get_tree(fc);
> -	if (ret < 0)
> -		goto out_psl;
> -
>  	if (resctrl_arch_alloc_capable())
>  		resctrl_arch_enable_alloc();
>  	if (resctrl_arch_mon_capable())
> @@ -2911,10 +2907,10 @@ static int rdt_get_tree(struct fs_context *fc)
>  						   RESCTRL_PICK_ANY_CPU);
>  	}
>  
> -	goto out;
> +	mutex_unlock(&rdtgroup_mutex);
> +	cpus_read_unlock();
> +	return kernfs_get_tree(fc);
>  
> -out_psl:
> -	rdt_pseudo_lock_release();
>  out_mondata:
>  	if (resctrl_arch_mon_capable())
>  		kernfs_remove(kn_mondata);
> 
> 
> This seems simpler by:
> * avoiding introduction of additional state (rdt_kill_sb_locked) with unclear protection,
> * avoiding double-cleanup on failure (rdt_kill_sb() called and then all rdt_get_tree()'s
>   failure path),
> * maintaining symmetry with rdt_kill_sb() by providing it the state it is
>   expected to be called with (i.e resctrl_mounted = true).

All these are excellent points in favor of this approach.
> 
> >From what I can tell it is safe to call kernfs_kill_sb() on failure of kernfs_get_tree(),
> but this needs to have been be considered as part of this submission anyway.

Looks OK to me too.

> Oh, maybe there is a new lock ordering issue with this that I am missing?

I can't see any lock issues.

But ... there is a problem. kernfs_get_tree() can fail for many reasons.
Only the specific case of failure in kernfs_get_super() makes the cleanup
call to rdt_kill_sb(). rdt_get_tree() has no way to tell from the error
code from kernfs_get_tree() whether cleanup has been done.

Plausibly I could do some surgery on the kernfs subsystem to make kernfs_get_tree()
take a second argument "bool *did_i_call_kill_sb". Only other user is
the cgroup code. So this might not be too invasive.

Or, I could fix up the comments to justify use of "resctrl_mount_in_progress"
Also fix up rdt_kill_sb() to look like this:

static void rdt_kill_sb(struct super_block *sb)
{
	if (resctrl_mount_in_progress) {
		resctrl_clean_up_failed_mount();
		return;
	}

	... existing unmount path code here ...
}

Or ... do you have some other suggestion?

> 
> Reinette

-Tony