shut down devices asynchronously

This adds the ability for the kernel to shutdown devices asynchronously.

Only devices with drivers that enable it are shut down asynchronously.

This can dramatically reduce system shutdown/reboot time on systems that
have multiple devices that take many seconds to shut down (like certain
NVMe drives). On one system tested, the shutdown time went from 11 minutes
without this patch to 55 seconds with the patch.

Changes from V9:

Address resource and timing issues when spawning a unique async thread
for every device during shutdown:
  * Make the asynchronous threads able to shut down multiple devices,
    instead of spawning a unique thread for every device.
  * Modify core kernel async code with a custom wake function so it
    doesn't wake up threads waiting to synchronize every time the cookie
    changes

Changes from V8:

Deal with shutdown hangs resulting when a parent/supplier device is
  later in the devices_kset list than its children/consumers:
  * Ignore sync_state_only devlinks for shutdown dependencies
  * Ignore shutdown_after for devices that don't want async shutdown
  * Add a sanity check to revert to sync shutdown for any device that
    would otherwise wait for a child/consumer shutdown that hasn't
    already been scheduled

Changes from V7:

Do not expose driver async_shutdown_enable in sysfs.
Wrapped a long line.
 
Changes from V6:

Removed a sysfs attribute that allowed the async device shutdown to be
"on" (with driver opt-out), "safe" (driver opt-in), or "off"... what was
previously "safe" is now the only behavior, so drivers now only need to
have the option to enable or disable async shutdown.

Changes from V5:

Separated into multiple patches to make review easier.
Reworked some code to make it more readable
Made devices wait for consumers to shut down, not just children
  (suggested by David Jeffery)

Changes from V4:

Change code to use cookies for synchronization rather than async domains
Allow async shutdown to be disabled via sysfs, and allow driver opt-in or
  opt-out of async shutdown (when not disabled), with ability to control
  driver opt-in/opt-out via sysfs

Changes from V3:

Bug fix (used "parent" not "dev->parent" in device_shutdown)

Changes from V2:

Removed recursive functions to schedule children to be shutdown before
  parents, since existing device_shutdown loop will already do this

Changes from V1:

Rewritten using kernel async code (suggested by Lukas Wunner)

David Jeffery (1):
  kernel/async: streamline cookie synchronization

Stuart Hayes (4):
  driver core: don't always lock parent in shutdown
  driver core: separate function to shutdown one device
  driver core: shut down devices asynchronously
  nvme-pci: Make driver prefer asynchronous shutdown

 drivers/base/base.h           |   8 ++
 drivers/base/core.c           | 210 +++++++++++++++++++++++++++++-----
 drivers/nvme/host/pci.c       |   1 +
 include/linux/device/driver.h |   2 +
 kernel/async.c                |  42 ++++++-
 5 files changed, 236 insertions(+), 27 deletions(-)

-- 
2.39.3