[RESEND,v2,0/5] block: skip media change event handling if unsupported

Message

Martin Wilck March 22, 2019, 10:43 p.m. UTC

(Resending upon request of Martin Petersen)

The block layer currently can't distinguish between gendisk devices that
don't support any media change events, and devices that do support them,
but only for internal purposes. Therefore the check_events() function is
called e.g. for ordinary non-removable SCSI disks. While these devices are
not polled under normal conditions, the check_events function is called on
certain synchronization points, in particular while the device is opened,
closed, or probed.

Under unfavorable conditions this can lead to processes being stalled on a
blocked queue: a close() schedules a work item for check_events() which
gets blocked in the work queue, and subsequent open() tries to flush the
workqueue. The flush then stalls too, as long as the the blocked work item
can't finish.

In principle, the gendisk->events field would make it very easy for the
block layer to check only for events actually supported by the device.
Currently this is impossible, because there are lots of drivers which don't
set gendisk->events although they implement the check_events() method.
This was introduced in commit 7c88a168da80 ("block: don't propagate unlisted 
DISK_EVENTs to userland") and follow-up patches because uevent generation by
these drivers was found to possibly generate infinite event loops between
kernel and user space. A side effect of these patches was that the
distinction between such devices and devices supporting no events at all
was lost.

This series implements a slightly different approach to event handling and
uevent suppression. The drivers are changed to set the events field again.
Whether or not uevents should be generated is controlled by a separate flag
bit, which is only set by the drivers that are known to generate proper
uevents (sd and sr). Once this is done, devices that don't support any
media change events can be clearly identified, and the whole events
checking code path can be skipped. This simplifies handling of
non-removable SCSI disks.

I have tested this with removable and non-removable SCSI disks, SCSI cdrom,
and ide-cd.

This patch set targets the same problem as Hannes' late submission "sd:
skip non-removable devices in sd_check_events()".

Changes in v2:

Removed the unused async_events field from struct gendisk.
This simplifies the event handling logic a bit.


Martin Wilck (5):
  block: genhd: remove async_events field
  block: disk_events: introduce event flags
  Revert "ide: unexport DISK_EVENT_MEDIA_CHANGE for ide-gd and ide-cd"
  Revert "block: unexport DISK_EVENT_MEDIA_CHANGE for legacy/fringe
    drivers"
  block: check_events: don't bother with events if unsupported

 block/genhd.c              | 51 ++++++++++++++++++++++----------------
 drivers/block/amiflop.c    |  1 +
 drivers/block/ataflop.c    |  1 +
 drivers/block/floppy.c     |  1 +
 drivers/block/paride/pcd.c |  1 +
 drivers/block/paride/pd.c  |  1 +
 drivers/block/paride/pf.c  |  1 +
 drivers/block/pktcdvd.c    |  1 -
 drivers/block/swim.c       |  1 +
 drivers/block/swim3.c      |  1 +
 drivers/block/xsysace.c    |  1 +
 drivers/cdrom/gdrom.c      |  1 +
 drivers/ide/ide-cd.c       |  1 +
 drivers/ide/ide-cd_ioctl.c |  5 ++--
 drivers/ide/ide-gd.c       |  6 +++--
 drivers/scsi/sd.c          |  3 ++-
 drivers/scsi/sr.c          |  3 ++-
 include/linux/genhd.h      |  8 +++++-
 18 files changed, 59 insertions(+), 29 deletions(-)