[lustre-devel] [PATCH v2] lustre: mdc: fix possible deadlock in chlg_open()

[James Simmons]

> Lockdep reports a possible deadlock between chlg_open() and
> mdc_changelog_cdev_init()
> 
> mdc_changelog_cdev_init() takes chlg_registered_dev_lock and then
> calls misc_register() which takes misc_mtx.
> chlg_open() is called while misc_mtx is held, and tries to take
> chlg_registered_dev_lock.
> If these two functions race, a deadlock can occur as each thread will
> hold one of the locks while trying to take the other.
> 
> chlg_open() does not need to take a lock.  It only uses the
> lock to stablize a list while looking for the matching
> chlg_registered_dev, and this can be found directly by examining
> file->private_data.
> 
> So remove chlg_obd_get(), and use file->private_data to find the
> obd_device.
> Also ensure the device is fully initialized before calling
> misc_register().  This means setting up some list linkage before the
> call, and tearing it down if there is an error.

I have been testing this but I'm no HSM expert. I pushed this patch
to OpenSFS branch as well.

https://jira.whamcloud.com/browse/LU-11617
https://review.whamcloud.com/#/c/33572/

Reviewed-by: James Simmons <jsimmons at infradead.org>

> Signed-off-by: NeilBrown <neilb at suse.com>
> ---
> 
> This is the revised version with the problem identified by Quentin
> fixed.
> 
>  drivers/staging/lustre/lustre/mdc/mdc_changelog.c | 46 +++++++----------------
>  1 file changed, 14 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/staging/lustre/lustre/mdc/mdc_changelog.c b/drivers/staging/lustre/lustre/mdc/mdc_changelog.c
> index d83507cbf95c..af29ea73c48a 100644
> --- a/drivers/staging/lustre/lustre/mdc/mdc_changelog.c
> +++ b/drivers/staging/lustre/lustre/mdc/mdc_changelog.c
> @@ -444,31 +444,6 @@ static ssize_t chlg_write(struct file *file, const char __user *buff,
>  	return rc < 0 ? rc : count;
>  }
>  
> -/**
> - * Find the OBD device associated to a changelog character device.
> - * @param[in]  cdev  character device instance descriptor
> - * @return corresponding OBD device or NULL if none was found.
> - */
> -static struct obd_device *chlg_obd_get(dev_t cdev)
> -{
> -	int minor = MINOR(cdev);
> -	struct obd_device *obd = NULL;
> -	struct chlg_registered_dev *curr;
> -
> -	mutex_lock(&chlg_registered_dev_lock);
> -	list_for_each_entry(curr, &chlg_registered_devices, ced_link) {
> -		if (curr->ced_misc.minor == minor) {
> -			/* take the first available OBD device attached */
> -			obd = list_first_entry(&curr->ced_obds,
> -					       struct obd_device,
> -					       u.cli.cl_chg_dev_linkage);
> -			break;
> -		}
> -	}
> -	mutex_unlock(&chlg_registered_dev_lock);
> -	return obd;
> -}
> -
>  /**
>   * Open handler, initialize internal CRS state and spawn prefetch thread if
>   * needed.
> @@ -479,12 +454,16 @@ static struct obd_device *chlg_obd_get(dev_t cdev)
>  static int chlg_open(struct inode *inode, struct file *file)
>  {
>  	struct chlg_reader_state *crs;
> -	struct obd_device *obd = chlg_obd_get(inode->i_rdev);
> +	struct miscdevice *misc = file->private_data;
> +	struct chlg_registered_dev *dev;
> +	struct obd_device *obd;
>  	struct task_struct *task;
>  	int rc;
>  
> -	if (!obd)
> -		return -ENODEV;
> +	dev = container_of(misc, struct chlg_registered_dev, ced_misc);
> +	obd = list_first_entry(&dev->ced_obds,
> +			       struct obd_device,
> +			       u.cli.cl_chg_dev_linkage);
>  
>  	crs = kzalloc(sizeof(*crs), GFP_KERNEL);
>  	if (!crs)
> @@ -669,13 +648,16 @@ int mdc_changelog_cdev_init(struct obd_device *obd)
>  		goto out_unlock;
>  	}
>  
> +	list_add_tail(&obd->u.cli.cl_chg_dev_linkage, &entry->ced_obds);
> +	list_add_tail(&entry->ced_link, &chlg_registered_devices);
> +
>  	/* Register new character device */
>  	rc = misc_register(&entry->ced_misc);
> -	if (rc != 0)
> +	if (rc != 0) {
> +		list_del_init(&obd->u.cli.cl_chg_dev_linkage);
> +		list_del(&entry->ced_link);
>  		goto out_unlock;
> -
> -	list_add_tail(&obd->u.cli.cl_chg_dev_linkage, &entry->ced_obds);
> -	list_add_tail(&entry->ced_link, &chlg_registered_devices);
> +	}
>  
>  	entry = NULL;	/* prevent it from being freed below */
>  
> -- 
> 2.14.0.rc0.dirty
> 
>