Re: [PATCH RFC] usb: gadget: f_tcm: fix circular lock dependency in usbg_make_tpg()

[Slawomir Stepien <sst@poczta.fm>]

On paź 13, 2026 01:38, 'syzbot' via syzkaller-upstream-moderation wrote:
> A circular locking dependency is detected by lockdep between the VFS
> inode_lock of configfs directories (&sb->s_type->i_mutex_key#24) and
> tpg_instances_lock during Target Portal Group (TPG) creation in
> usbg_make_tpg():
> 
> WARNING: possible circular locking dependency detected
> ...
> task is trying to acquire lock:
> ffffffff8fc6d8c8 (gadget_subsys.su_mutex){+.+.}-{4:4}, at:
> uvcg_extension_allow_link+0x6c/0x1f0
> drivers/usb/gadget/function/uvc_configfs.c:1195
> 
> but task is already holding lock:
> ffff8881f3ef6280 (&sb->s_type->i_mutex_key#24){+.+.}-{4:4}, at: inode_lock
> include/linux/fs.h:1024 [inline]
> ffff8881f3ef6280 (&sb->s_type->i_mutex_key#24){+.+.}-{4:4}, at:
> configfs_symlink+0x4d0/0x11d0 fs/configfs/symlink.c:201
> 
> which lock already depends on the new lock.
> 
> the existing dependency chain (in reverse order) is:
> 
>  -> #3 (&sb->s_type->i_mutex_key#24){+.+.}-{4:4}:
>         inode_lock include/linux/fs.h:1024 [inline]
>         configfs_depend_item_unlocked+0x153/0x420 fs/configfs/dir.c:1268
>         usbg_make_tpg+0x1e0/0x550 drivers/usb/gadget/function/f_tcm.c:1692
>         target_fabric_make_tpg+0xa8/0x6d0
>         drivers/target/target_core_fabric_configfs.c:947
>         configfs_mkdir+0x4f6/0x9e0 fs/configfs/dir.c:1369
> 
>  -> #2 (tpg_instances_lock){+.+.}-{4:4}:
>         tcm_alloc_inst+0x65/0x1a0 drivers/usb/gadget/function/f_tcm.c:2687
>         try_get_usb_function_instance drivers/usb/gadget/functions.c:28
>         [inline]
>         usb_get_function_instance+0xe3/0x2f0
>         drivers/usb/gadget/functions.c:44
>         function_make+0x127/0x360 drivers/usb/gadget/configfs.c:626
>         configfs_mkdir+0x4f6/0x9e0 fs/configfs/dir.c:1369
> 
>  -> #1 (func_lock){+.+.}-{4:4}:
>         try_get_usb_function_instance drivers/usb/gadget/functions.c:18
>         [inline]
>         usb_get_function_instance+0x2e/0x2f0
>         drivers/usb/gadget/functions.c:44
>         function_make+0x127/0x360 drivers/usb/gadget/configfs.c:626
>         configfs_mkdir+0x4f6/0x9e0 fs/configfs/dir.c:1369
> 
>  -> #0 (gadget_subsys.su_mutex){+.+.}-{4:4}:
>         uvcg_extension_allow_link+0x6c/0x1f0
>         drivers/usb/gadget/function/uvc_configfs.c:1195
>         configfs_symlink+0x74e/0x11d0 fs/configfs/symlink.c:211
> 
> Chain exists of:
>   gadget_subsys.su_mutex --> tpg_instances_lock -->
>   &sb->s_type->i_mutex_key#24
> 
> The dependency chain arises because during gadget function instantiation
> (such as creating a tcm.usb0 function directory), the VFS layer acquires
> the parent directory inode_lock (&sb->s_type->i_mutex_key#24).
> configfs_mkdir() then acquires gadget_subsys.su_mutex and calls
> function_make(). This invokes usb_get_function_instance(), which acquires
> func_lock and calls tcm_alloc_inst(), which in turn acquires
> tpg_instances_lock. This establishes the locking order: inode_lock ->
> gadget_subsys.su_mutex -> func_lock -> tpg_instances_lock. Conversely, when
> creating a Target Portal Group (TPG) in the target subsystem,
> target_fabric_make_tpg() calls usbg_make_tpg(), which acquires
> tpg_instances_lock and then calls configfs_depend_item_unlocked(). Because
> the target configfs item belongs to a different subsystem (gadget_subsys),
> configfs_depend_item_unlocked() acquires the configfs root directory
> inode_lock. Since all configfs directories share the same inode_lock class
> in lockdep, this creates a circular dependency: inode_lock ->
> gadget_subsys.su_mutex -> func_lock -> tpg_instances_lock -> inode_lock.
> 
> To break this circular dependency, drop tpg_instances_lock around the call
> to configfs_depend_item_unlocked(). This unlock is strictly confined to the
> !opts->has_dep branch where configfs_depend_item_unlocked() is invoked,
> since the opts->has_dep path only calls try_module_get() and does not
> acquire configfs inode locks. Before dropping tpg_instances_lock, pin the
> item via config_item_get() to prevent concurrent release or removal. After
> configfs_depend_item_unlocked() completes, release the reference with
> config_item_put() before re-acquiring tpg_instances_lock so that an
> eventual cleanup cannot deadlock on tpg_instances_lock. Finally, once
> tpg_instances_lock is re-acquired, verify that the instance slot remains
> valid and unassigned (tpg_instances[i].func_inst == &opts->func_inst &&
> !tpg_instances[i].tpg); if another thread claimed or changed the slot while
> unlocked, unwind the dependency with configfs_undepend_item_unlocked() and
> return -EBUSY.
> 
> Fixes: 9dbf74f4022f ("usb: gadget: f_tcm: fix deadlock in usbg_make_tpg()")
> Assisted-by: Gemini:gemini-3.8-flash Gemini:gemini-3.1-pro-preview syzbot
> Reported-by: syzbot+b3410eb576b568216ae5@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b3410eb576b568216ae5
> Link: https://syzkaller.appspot.com/ai_job?id=7da59cc9-b02a-4e8b-a07f-d2df215094c6
> To: "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>
> To: <linux-usb@vger.kernel.org>
> To: "Yun Zhou" <yun.zhou@windriver.com>
> Cc: "Jiasheng Jiang" <jiashengjiangcool@gmail.com>
> Cc: <linux-kernel@vger.kernel.org>
> Cc: "Cen Zhang" <zzzccc427@gmail.com>
> 
> ---
> diff --git a/drivers/usb/gadget/function/f_tcm.c b/drivers/usb/gadget/function/f_tcm.c
> index 9e6d4f399..e2399b4cc 100644
> --- a/drivers/usb/gadget/function/f_tcm.c
> +++ b/drivers/usb/gadget/function/f_tcm.c
> @@ -1655,6 +1655,7 @@ static struct se_portal_group *usbg_make_tpg(struct se_wwn *wwn,
>  {
>  	struct usbg_tport *tport = container_of(wwn, struct usbg_tport,
>  			tport_wwn);
> +	struct config_item *item;
>  	struct usbg_tpg *tpg;
>  	u16 tpgt;
>  	int ret;
> @@ -1682,18 +1683,29 @@ static struct se_portal_group *usbg_make_tpg(struct se_wwn *wwn,
>  		if (!try_module_get(opts->dependent))
>  			goto unlock_inst;
>  	} else {
> +		item = config_item_get(&opts->func_inst.group.cg_item);
> +		mutex_unlock(&tpg_instances_lock);
> +
>  		/*
>  		 * configfs_depend_item_unlocked() may acquire the configfs
>  		 * root inode lock when the target belongs to a different
> -		 * subsystem. Calling it under dep_lock would create a
> -		 * circular dependency:
> -		 *   dep_lock -> configfs inode lock -> su_mutex -> dep_lock
> +		 * subsystem. Calling it under tpg_instances_lock would create
> +		 * a circular dependency:
> +		 *   inode_lock -> su_mutex -> func_lock -> tpg_instances_lock -> inode_lock

Altering this comment in this way is a mistake in my view. Comment about the dep_lock should still
be visible - it is still valid.

If you want to comment about tpg_instances_lock add it before unlocking the lock above or rephrase
this comment block such that both locks are mentioned.

>  		 */
>  		ret = configfs_depend_item_unlocked(
>  			wwn->wwn_group.cg_subsys,
> -			&opts->func_inst.group.cg_item);
> +			item);
> +		config_item_put(item);
> +
> +		mutex_lock(&tpg_instances_lock);
>  		if (ret)
>  			goto unlock_inst;
> +		if (tpg_instances[i].func_inst != &opts->func_inst ||
> +		    tpg_instances[i].tpg) {
> +			ret = -EBUSY;
> +			goto unref_dep;
> +		}
>  	}
>  
>  	tpg = kzalloc_obj(struct usbg_tpg);
> 
> 
> base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907

-- 
Slawomir Stepien