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

[NeilBrown <neilb@suse.com>]

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.

Signed-off-by: NeilBrown <neilb@suse.com>
---

Note that this bug exists in upstream (OpenSFS) lustre as well.

 drivers/staging/lustre/lustre/mdc/mdc_changelog.c | 35 +++++------------------
 1 file changed, 7 insertions(+), 28 deletions(-)

diff --git a/drivers/staging/lustre/lustre/mdc/mdc_changelog.c b/drivers/staging/lustre/lustre/mdc/mdc_changelog.c
index d83507cbf95c..645e7f24a859 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)
-- 
2.14.0.rc0.dirty

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