implement in-kernel gendisk events handling

Currently, media presence polling for removeable block devices is done
from userland.  There are several issues with this.

* Polling is done by periodically opening the device.  For SCSI
  devices, the command sequence generated by such action involves a
  few different commands including TEST_UNIT_READY.  This behavior,
  while perfectly legal, is different from Windows which only issues
  single command, GET_EVENT_STATUS_NOTIFICATION.  Unfortunately, some
  ATAPI devices lock up after being periodically queried such command
  sequences.

* There is no reliable and unintrusive way for a userland program to
  tell whether the target device is safe for media presence polling.
  For example, polling for media presence during an on-going burning
  session can make it fail.  The polling program can avoid this by
  opening the device with O_EXCL but then it risks making a valid
  exclusive user of the device fail w/ -EBUSY.

* Userland polling is unnecessarily heavy and in-kernel implementation
  is lighter and better coordinated (workqueue, timer slack).

This patch implements framework for in-kernel disk event handling,
which includes media presence polling.

* bdops->check_events() is added, which supercedes ->media_changed().
  It should check whether there's any pending event and return if so.
  Currently, two events are defined - DISK_EVENT_MEDIA_CHANGE and
  DISK_EVENT_EJECT_REQUEST.  ->check_events() is guaranteed not to be
  called parallelly.

* gendisk->events and ->async_events are added.  These should be
  initialized by block driver before passing the device to add_disk().
  The former contains the mask of all supported events and the latter
  the mask of all events which the device can report without polling.
  /sys/block/*/events[_async] export these to userland.

* Kernel parameter block.events_dfl_poll_msecs controls the system
  polling interval (default is 0 which means disable) and
  /sys/block/*/events_poll_msecs control polling intervals for
  individual devices (default is -1 meaning use system setting).  Note
  that if a device can report all supported events asynchronously and
  its polling interval isn't explicitly set, the device won't be
  polled regardless of the system polling interval.

* If a device is opened exclusively with write access, event checking
  is automatically disabled until all write exclusive accesses are
  released.

* There are event 'clearing' events.  For example, both of currently
  defined events are cleared after the device has been successfully
  opened.  This information is passed to ->check_events() callback
  using @clearing argument as a hint.

* Event checking is always performed from system_nrt_wq and timer
  slack is set to 25% for polling.

* Nothing changes for drivers which implement ->media_changed() but
  not ->check_events().  Going forward, all drivers will be converted
  to ->check_events() and ->media_change() will be dropped.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 34 insertions, 7 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index c1c1b8c3fb99..6017389711ee 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -948,10 +948,11 @@ int check_disk_change(struct block_device *bdev)
 {
         struct gendisk *disk = bdev->bd_disk;
         const struct block_device_operations *bdops = disk->fops;
+        unsigned int events;
 
-        if (!bdops->media_changed)
-                return 0;
-        if (!bdops->media_changed(bdev->bd_disk))
+        events = disk_clear_events(disk, DISK_EVENT_MEDIA_CHANGE |
+                                   DISK_EVENT_EJECT_REQUEST);
+        if (!(events & DISK_EVENT_MEDIA_CHANGE))
                 return 0;
 
         flush_disk(bdev);
@@ -1158,9 +1159,10 @@ int blkdev_get(struct block_device *bdev, fmode_t mode, void *holder)
 
         if (whole) {
                 /* finish claiming */
+                mutex_lock(&bdev->bd_mutex);
                 spin_lock(&bdev_lock);
 
-                if (res == 0) {
+                if (!res) {
                         BUG_ON(!bd_may_claim(bdev, whole, holder));
                         /*
                          * Note that for a whole device bd_holders
@@ -1180,6 +1182,20 @@ int blkdev_get(struct block_device *bdev, fmode_t mode, void *holder)
                 wake_up_bit(&whole->bd_claiming, 0);
 
                 spin_unlock(&bdev_lock);
+
+                /*
+                 * Block event polling for write claims.  Any write
+                 * holder makes the write_holder state stick until all
+                 * are released.  This is good enough and tracking
+                 * individual writeable reference is too fragile given
+                 * the way @mode is used in blkdev_get/put().
+                 */
+                if (!res && (mode & FMODE_WRITE) && !bdev->bd_write_holder) {
+                        bdev->bd_write_holder = true;
+                        disk_block_events(bdev->bd_disk);
+                }
+
+                mutex_unlock(&bdev->bd_mutex);
                 bdput(whole);
         }
 
@@ -1353,12 +1369,23 @@ int blkdev_put(struct block_device *bdev, fmode_t mode)
 
                 spin_unlock(&bdev_lock);
 
-                /* if this was the last claim, holder link should go too */
-                if (bdev_free)
+                /*
+                 * If this was the last claim, remove holder link and
+                 * unblock evpoll if it was a write holder.
+                 */
+                if (bdev_free) {
                         bd_unlink_disk_holder(bdev);
+                        if (bdev->bd_write_holder) {
+                                disk_unblock_events(bdev->bd_disk);
+                                bdev->bd_write_holder = false;
+                        } else
+                                disk_check_events(bdev->bd_disk);
+                }
 
                 mutex_unlock(&bdev->bd_mutex);
-        }
+        } else
+                disk_check_events(bdev->bd_disk);
+
         return __blkdev_put(bdev, mode, 0);
 }
 EXPORT_SYMBOL(blkdev_put);