Re: [PATCH v2] cxl/region: Fix a race bug in delete_region_store

[Dave Jiang]

On 4/21/26 9:56 PM, Sungwoo Kim wrote:
> devm_release_action() cannot find a matching entry in the following two
> race scenarios.
> 
> scenario 1: delete two same regions concurrently
> 
> CPU 0                     CPU 1
> ======                    ======
> delete_region_store()
>   cxlr = cxl_find_region_by_name()
>                           delete_region_store()
>                             cxlr = cxl_find_region_by_name()
>   devm_release_action()
>                             devm_release_action()
>                             // cannot find the action, WARN_ON()
> 
> scenario 2: delete parent and child concurrently [1]
> 
> CPU0                                    CPU1
> devres_release_all()
>    // take devres_lock
>    remove_nodes(devres_head) // mv to local todo
>    // drop devres_lock                   delete_region_store()
>                                            cxlr = cxl_find_region_by_name()  // success
>                                            devm_release_action(unregister_region)
>                                              devres_release()
>                                                devres_remove()
>                                                  // hold devres_lock
>                                                  find_dr(devres_head) // does not find it
>                                              WARN_ON(-ENOENT)
>    release_nodes() // drain todo
>      unregister_region(cxlr) // release() cb
>        device_del()
> 
> To fix scenario 1, delete_region_store() directly calls
> unregister_region() with a test_and_set_bit(CXL_REGION_F_UNREGISTER).
> Also, replace devm_release_action() to devm_remove_action() as
> unregister_region() is now called directly.
> 
> To fix scenario 2, delete_region_store() removes actions only if the
> driver is still attached. To ensure this, scoped_guard(device,
> port->uport_dev) is required to check port->uport_dev->driver.
> To hold this lock, a workqueue is required for a clean context.
> 
> Splat:
> 
> WARNING: drivers/base/devres.c:824 at devm_release_action drivers/base/devres.c:824 [inline], CPU#0: syz.1.12224/47589
> WARNING: drivers/base/devres.c:824 at devm_release_action+0x2b2/0x360 drivers/base/devres.c:817, CPU#0: syz.1.12224/47589
> 
> [1] https://lore.kernel.org/linux-cxl/20260310183644.4rwc7ilmzy4t5xp6@offworld/
> 
> Fixes: 779dd20cfb56 ("cxl/region: Add region creation support")
> Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Signed-off-by: Sungwoo Kim <iam@xxxxxxxxxxxx>

Sungwoo,
I saw this on sashiko but have not reviewed the validity. Maybe take a look.
https://sashiko.dev/#/patchset/20260422045637.3048249-2-iam%40sung-woo.kim

DJ

> ---
> V1: https://lore.kernel.org/linux-cxl/20260308185958.2453707-2-iam@xxxxxxxxxxxx/
> V1->V2:
> - Made devm_remove_action() asynchronous.
> - Made unregister_region() idempotent.
> - Addressed Dan's comments and added the suggested-by tag.
> 
>  drivers/cxl/core/region.c | 45 ++++++++++++++++++++++++++++++++++++---
>  drivers/cxl/cxl.h         |  8 +++++++
>  2 files changed, 50 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index e50dc716d4e8..64db0d332c13 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -39,6 +39,7 @@
>  static nodemask_t nodemask_region_seen = NODE_MASK_NONE;
>  
>  static struct cxl_region *to_cxl_region(struct device *dev);
> +static void remove_devm_actions_work(struct work_struct *work);
>  
>  #define __ACCESS_ATTR_RO(_level, _name) {				\
>  	.attr	= { .name = __stringify(_name), .mode = 0444 },		\
> @@ -2543,6 +2544,9 @@ static void unregister_region(void *_cxlr)
>  	struct cxl_region_params *p = &cxlr->params;
>  	int i;
>  
> +	if (test_and_set_bit(CXL_REGION_F_UNREGISTER, &cxlr->flags))
> +		return;
> +
>  	device_del(&cxlr->dev);
>  
>  	/*
> @@ -2589,6 +2593,8 @@ static struct cxl_region *cxl_region_alloc(struct cxl_root_decoder *cxlrd, int i
>  	dev->type = &cxl_region_type;
>  	cxl_region_setup_flags(cxlr, &cxlrd->cxlsd.cxld);
>  
> +	INIT_WORK(&cxlr->remove_work, remove_devm_actions_work);
> +
>  	return cxlr;
>  }
>  
> @@ -2831,20 +2837,53 @@ cxl_find_region_by_name(struct cxl_root_decoder *cxlrd, const char *name)
>  	return to_cxl_region(region_dev);
>  }
>  
> +static void remove_devm_actions_work(struct work_struct *work)
> +{
> +	struct cxl_region *cxlr = container_of(work, typeof(*cxlr), remove_work);
> +	struct cxl_root_decoder *cxlrd = cxlr->cxlrd;
> +	struct cxl_port *port = to_cxl_port(cxlrd->cxlsd.cxld.dev.parent);
> +
> +	if (test_and_set_bit(CXL_REGION_F_DEVM_REMOVE, &cxlr->flags)) {
> +		put_device(&cxlr->dev);
> +		return;
> +	}
> +
> +	scoped_guard(device, port->uport_dev) {
> +		if (port->uport_dev->driver)
> +			devm_remove_action(port->uport_dev, unregister_region, cxlr);
> +	}
> +
> +	put_device(&cxlr->dev);
> +}
> +
> +static int remove_devm_actions(struct cxl_region *cxlr)
> +{
> +	if (!schedule_work(&cxlr->remove_work))
> +		return -EBUSY;
> +
> +	return 0;
> +}
> +
>  static ssize_t delete_region_store(struct device *dev,
>  				   struct device_attribute *attr,
>  				   const char *buf, size_t len)
>  {
>  	struct cxl_root_decoder *cxlrd = to_cxl_root_decoder(dev);
> -	struct cxl_port *port = to_cxl_port(dev->parent);
>  	struct cxl_region *cxlr;
> +	int rc;
>  
> +	/* remove_devm_actions_work() will put cxlr->dev. */
>  	cxlr = cxl_find_region_by_name(cxlrd, buf);
>  	if (IS_ERR(cxlr))
>  		return PTR_ERR(cxlr);
>  
> -	devm_release_action(port->uport_dev, unregister_region, cxlr);
> -	put_device(&cxlr->dev);
> +	unregister_region(cxlr);
> +
> +	rc = remove_devm_actions(cxlr);
> +	if (rc) {
> +		put_device(&cxlr->dev);
> +		return rc;
> +	}
>  
>  	return len;
>  }
> diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> index 1297594beaec..75ec292a9f42 100644
> --- a/drivers/cxl/cxl.h
> +++ b/drivers/cxl/cxl.h
> @@ -447,6 +447,12 @@ struct cxl_region_params {
>   */
>  #define CXL_REGION_F_NORMALIZED_ADDRESSING 3
>  
> +/* Indicate that this region is being unregistered to prevent a race. */
> +#define CXL_REGION_F_UNREGISTER 4
> +
> +/* Indicate that this region called devm_remove_action. */
> +#define CXL_REGION_F_DEVM_REMOVE 5
> +
>  /**
>   * struct cxl_region - CXL region
>   * @dev: This region's device
> @@ -462,6 +468,7 @@ struct cxl_region_params {
>   * @coord: QoS access coordinates for the region
>   * @node_notifier: notifier for setting the access coordinates to node
>   * @adist_notifier: notifier for calculating the abstract distance of node
> + * @remove_work: trigger the remove action in a safe context to acquire locks
>   */
>  struct cxl_region {
>  	struct device dev;
> @@ -477,6 +484,7 @@ struct cxl_region {
>  	struct access_coordinate coord[ACCESS_COORDINATE_MAX];
>  	struct notifier_block node_notifier;
>  	struct notifier_block adist_notifier;
> +	struct work_struct remove_work;
>  };
>  
>  struct cxl_nvdimm_bridge {