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 v3:
- Improved formatting of commit messages (Christoph)
- 2/5: Rather than adding the event flag bits to the "events" field,
add a new field "event_flags" to struct gendisk (Christoph)
- 2/5: Add a pair of parentheses around flag test (Christoph)
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 | 48 ++++++++++++++++++++++----------------
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 | 1 +
drivers/scsi/sr.c | 1 +
include/linux/genhd.h | 11 +++++++--
18 files changed, 57 insertions(+), 27 deletions(-)
--
2.21.0
