s390/dasd: Fix locking issue when changing RO attribute

The function dasd_ro_store() calls set_disk_ro() to set the device in
question read-only. Since set_disk_ro() might sleep, we can't call it
while holding a lock. However, we also can't simply check if the device,
block, and gdp references are valid before we call set_disk_ro() because
an offline processing might have been started in the meanwhile which
will destroy those references.

In order to reliably call set_disk_ro() we have to ensure several
things:

- Still check validity of the mentioned references but additionally
  check if offline processing is running and bail out accordingly. Also,
  do this while holding the device lock.
- To ensure that the block device is still safe after the lock, increase
  the open_count while still holding the device lock.

Reviewed-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/drivers/s390/block/dasd_devmap.c b/drivers/s390/block/dasd_devmap.c
index c1979ec..cbae6ab 100644 (file)
--- a/drivers/s390/block/dasd_devmap.c
+++ b/drivers/s390/block/dasd_devmap.c
@@ -761,6 +761,7 @@ dasd_ro_store(struct device *dev, struct device_attribute *attr,
 {
        struct ccw_device *cdev = to_ccwdev(dev);
        struct dasd_device *device;
+       unsigned long flags;
        unsigned int val;
        int rc;
 
@@ -775,10 +776,22 @@ dasd_ro_store(struct device *dev, struct device_attribute *attr,
        if (IS_ERR(device))
                return PTR_ERR(device);
 
+       spin_lock_irqsave(get_ccwdev_lock(cdev), flags);
        val = val || test_bit(DASD_FLAG_DEVICE_RO, &device->flags);
-       if (device->block && device->block->gdp)
-               set_disk_ro(device->block->gdp, val);
 
+       if (!device->block || !device->block->gdp ||
+           test_bit(DASD_FLAG_OFFLINE, &device->flags)) {
+               spin_unlock_irqrestore(get_ccwdev_lock(cdev), flags);
+               goto out;
+       }
+       /* Increase open_count to avoid losing the block device */
+       atomic_inc(&device->block->open_count);
+       spin_unlock_irqrestore(get_ccwdev_lock(cdev), flags);
+
+       set_disk_ro(device->block->gdp, val);
+       atomic_dec(&device->block->open_count);
+
+out:
        dasd_put_device(device);
 
        return count;