Fix using zvol as slog device

During the original ZoL port the vdev_uses_zvols() function was
disabled until it could be properly implemented.  This prevented
a zpool from use a zvol for its slog device.

This patch implements that missing functionality by adding a
zvol_is_zvol() function to zvol.c.  Given the full path to a
device it will lookup the device and verify its major number
against the registered zvol major number for the system.  If
they match we know the device is a zvol.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1131
Set elevator for DM devices despite vdev_wholedisk

The current state of udev and devicer-mapper devices makes it difficult
to construct a mapping of DM partitions and their underlying DM device.
For example, with a /dev directory with the following contents:

    $ ls -d /dev/dm-*
    /dev/dm-0
    /dev/dm-1
    /dev/dm-2
    /dev/dm-3

it is not immediately apparent if these are completely separate devices,
or partitions and real devices intermixed. In contrast, SCSI devices
would appear as so:

    $ ls -d /dev/sd*
    /dev/sda
    /dev/sda1
    /dev/sdb
    /dev/sdb1

Here, one can immediately determine that there are two devices (sda and
sdb), each containing a single partition. The lack of a predictable and
consistent mapping from DM devices to DM device partitions makes it
difficult for user space to process these devices the same way it does
SCSI devices.

As a result, the ZFS utilities do not partition DM devices, and instead
set the "vdev_wholedisk" label to 0 and treat them as partitions. This
has the side effect that, even if ZFS has sole ownership of the device,
the IO scheduler will not be modified because it is treated as a
partition.

This change adds an exception for DM devices in vdev_elevator_switch,
allowing the elevator to be modified even though the "vdev_wholedisk"
property is not set.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1149
Remove TSD zfs_fsyncer_key

It's my understanding that the zfs_fsyncer_key TSD was added as
a performance omtimization to reduce contention on the zl_lock
from zil_commit().  This issue manifested itself as very long
(100+ms) fsync() system call times for fsync() heavy workloads.

However, under Linux I'm not seeing the same contention that
was originally described.  Therefore, I'm removing this code
in order to ween ourselves off any dependence on TSD.  If the
original performance issue reappears on Linux we can revisit
fixing it without resorting to TSD.

This just leaves one small ZFS TSD consumer.  If it can be
cleanly removed from the code we'll be able to shed the SPL
TSD implementation entirely.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/spl#174
Remove the ChangeLog

The ChangeLog was retired long ago, the git commit logs are
authoritative.  To avoid any confusion remove the ChangeLog.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Refresh AUTHORS

The AUTHORS file was getting stale.  Refresh its contents
using the authors listed in the git commit logs.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Revert "Remove TSD zfs_fsyncer_key"

This reverts commit 31f2b5abdf95d8426d8bfd66ca7f62ec70215e3c back
to the original code until the fsync(2) performance regression
can be addressed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Fix zpool on zvol lock inversion deadlock

In all but one case the spa_namespace_lock is taken before the
bdev->bd_mutex lock.  But Linux __blkdev_get() function calls
fops->open() with the bdev->bd_mutex lock held and we must
somehow still safely acquire the spa_namespace_lock.

To avoid a potential lock inversion deadlock we preemptively
try to take the spa_namespace_lock().  Normally it will not
be contended and this is safe because spa_open_common() handles
the case where the caller already holds the spa_namespace_lock.

When it is contended we risk a lock inversion if we were to
block waiting for the lock.  Luckily, the __blkdev_get()
function allows us to return -ERESTARTSYS which will result in
bdev->bd_mutex being dropped, reacquired, and fops->open() being
called again.  This process can be repeated safely until both
locks are acquired.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #612
ZFS 0.6.0-rc13
Merge branch 'master' of git://github.com/zfsonlinux/zfs into feature-import