slow-work doesn't have any user left.  Kill it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NDavid Howells <dhowells@redhat.com>

Workqueue can now handle high concurrency.  Convert gfs to use
workqueue instead of slow-work.

* Steven pointed out that recovery path might be run from allocation
  path and thus requires forward progress guarantee without memory
  allocation.  Create and use gfs_recovery_wq with rescuer.  Please
  note that forward progress wasn't guaranteed with slow-work.

* Updated to use non-reentrant workqueue.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NSteven Whitehouse <swhiteho@redhat.com>

Workqueue can now handle high concurrency.  Convert drm_crtc_helper to
use system_nrt_wq instead of slow-work.  The conversion is mostly
straight forward.  One difference is that drm_helper_hpd_irq_event()
no longer blocks and can be called from any context.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NDavid Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org

Workqueue can now handle high concurrency.  Use system_nrt_wq
instead of slow-work.

* Updated is_valid_oplock_break() to not call cifs_oplock_break_put()
  as advised by Steve French.  It might cause deadlock.  Instead,
  reference is increased after queueing succeeded and
  cifs_oplock_break() briefly grabs GlobalSMBSeslock before putting
  the cfile to make sure it doesn't put before the matching get is
  finished.

* Anton Blanchard reported that cifs conversion was using now gone
  system_single_wq.  Use system_nrt_wq which provides non-reentrance
  guarantee which is enough and much better.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NSteve French <sfrench@samba.org>
Cc: Anton Blanchard <anton@samba.org>

fscache no longer uses slow-work.  Drop references to it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NDavid Howells <dhowells@redhat.com>

Make fscache operation to use only workqueue instead of combination of
workqueue and slow-work.  FSCACHE_OP_SLOW is dropped and
FSCACHE_OP_FAST is renamed to FSCACHE_OP_ASYNC and uses newly added
fscache_op_wq workqueue to execute op->processor().
fscache_operation_init_slow() is dropped and fscache_operation_init()
now takes @processor argument directly.

* Unbound workqueue is used.

* fscache_retrieval_work() is no longer necessary as OP_ASYNC now does
  the equivalent thing.

* sysctl fscache.operation_max_active added to control concurrency.
  The default value is nr_cpus clamped between 2 and
  WQ_UNBOUND_MAX_ACTIVE.

* debugfs support is dropped for now.  Tracing API based debug
  facility is planned to be added.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NDavid Howells <dhowells@redhat.com>

Make fscache object state transition callbacks use workqueue instead
of slow-work.  New dedicated unbound CPU workqueue fscache_object_wq
is created.  get/put callbacks are renamed and modified to take
@object and called directly from the enqueue wrapper and the work
function.  While at it, make all open coded instances of get/put to
use fscache_get/put_object().

* Unbound workqueue is used.

* work_busy() output is printed instead of slow-work flags in object
  debugging outputs.  They mean basically the same thing bit-for-bit.

* sysctl fscache.object_max_active added to control concurrency.  The
  default value is nr_cpus clamped between 4 and
  WQ_UNBOUND_MAX_ACTIVE.

* slow_work_sleep_till_thread_needed() is replaced with fscache
  private implementation fscache_object_sleep_till_congested() which
  waits on fscache_object_wq congestion.

* debugfs support is dropped for now.  Tracing API based debug
  facility is planned to be added.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NDavid Howells <dhowells@redhat.com>

Once a work starts execution, its data contains the cpu number it was
on instead of pointing to cwq.  This is added by commit 7a22ad75
(workqueue: carry cpu number in work data once execution starts) to
reliably determine the work was last on even if the workqueue itself
was destroyed inbetween.

Whether data points to a cwq or contains a cpu number was
distinguished by comparing the value against PAGE_OFFSET.  The
assumption was that a cpu number should be below PAGE_OFFSET while a
pointer to cwq should be above it.  However, on architectures which
use separate address spaces for user and kernel spaces, this doesn't
hold as PAGE_OFFSET is zero.

Fix it by using an explicit flag, WORK_STRUCT_CWQ, to mark what the
data field contains.  If the flag is set, it's pointing to a cwq;
otherwise, it contains a cpu number.

Reported on s390 and microblaze during linux-next testing.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NSachin Sant <sachinp@in.ibm.com>
Reported-by: NMichal Simek <michal.simek@petalogix.com>
Reported-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>
Tested-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>
Tested-by: NMichal Simek <monstr@monstr.eu>

All cpumasks are assumed to have cpu 0 permanently set on UP, so it
can't be used to signify whether there's something to be done for the
CPU.  workqueue was using cpumask to track which CPU requested rescuer
assistance and this led rescuer thread to think there always are
pending mayday requests on UP, which resulted in infinite busy loops.

This patch fixes the problem by introducing mayday_mask_t and
associated helpers which wrap cpumask on SMP and emulates its behavior
using bitops and unsigned long on UP.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>

Commit f3421797 (workqueue: implement unbound workqueue) incorrectly
tested CONFIG_SMP as part of a C expression in alloc/free_cwqs().  As
CONFIG_SMP is not defined in UP, this breaks build.  Fix it by using

Found during linux-next build test.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>

maybe_create_worker() mismanaged locking when worker creation fails
and it has to retry.  Fix locking and simplify lock manipulation.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NYong Zhang <yong.zhang@windriver.com>

Replace private worker pool with system_unbound_wq.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NArjan van de Ven <arjan@infradead.org>

WQ_SINGLE_CPU combined with @max_active of 1 is used to achieve full
ordering among works queued to a workqueue.  The same can be achieved
using WQ_UNBOUND as unbound workqueues always use the gcwq for
WORK_CPU_UNBOUND.  As @max_active is always one and benefits from cpu
locality isn't accessible anyway, serving them with unbound workqueues
should be fine.

Drop WQ_SINGLE_CPU support and use WQ_UNBOUND instead.  Note that most
single thread workqueue users will be converted to use multithread or
non-reentrant instead and only the ones which require strict ordering
will keep using WQ_UNBOUND + @max_active of 1.
Signed-off-by: NTejun Heo <tj@kernel.org>

This patch implements unbound workqueue which can be specified with
WQ_UNBOUND flag on creation.  An unbound workqueue has the following
properties.

* It uses a dedicated gcwq with a pseudo CPU number WORK_CPU_UNBOUND.
  This gcwq is always online and disassociated.

* Workers are not bound to any CPU and not concurrency managed.  Works
  are dispatched to workers as soon as possible and the only applied
  limitation is @max_active.  IOW, all unbound workqeueues are
  implicitly high priority.

Unbound workqueues can be used as simple execution context provider.
Contexts unbound to any cpu are served as soon as possible.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: David Howells <dhowells@redhat.com>

In preparation of WQ_UNBOUND addition, make the following changes.

* Add WORK_CPU_* constants for pseudo cpu id numbers used (currently
  only WORK_CPU_NONE) and use them instead of NR_CPUS.  This is to
  allow another pseudo cpu id for unbound cpu.

* Reorder WQ_* flags.

* Make workqueue_struct->cpu_wq a union which contains a percpu
  pointer, regular pointer and an unsigned long value and use
  kzalloc/kfree() in UP allocation path.  This will be used to
  implement unbound workqueues which will use only one cwq on SMPs.

* Move alloc_cwqs() allocation after initialization of wq fields, so
  that alloc_cwqs() has access to wq->flags.

* Trivial relocation of wq local variables in freeze functions.

These changes don't cause any functional change.
Signed-off-by: NTejun Heo <tj@kernel.org>

libata has two concurrency related limitations.

a. ata_wq which is used for polling PIO has single thread per CPU.  If
   there are multiple devices doing polling PIO on the same CPU, they
   can't be executed simultaneously.

b. ata_aux_wq which is used for SCSI probing has single thread.  In
   cases where SCSI probing is stalled for extended period of time
   which is possible for ATAPI devices, this will stall all probing.

#a is solved by increasing maximum concurrency of ata_wq.  Please note
that polling PIO might be used under allocation path and thus needs to
be served by a separate wq with a rescuer.

#b is solved by using the default wq instead and achieving exclusion
via per-port mutex.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NJeff Garzik <jgarzik@pobox.com>

When there's no pending work to do, worker_thread() goes back to sleep
after waking up without checking whether worker management is
necessary.  This means that idle worker exit requests can be ignored
if the gcwq stays empty.

Fix it by making worker_thread() always check whether worker
management is necessary before going to sleep.
Signed-off-by: NTejun Heo <tj@kernel.org>

get_work_gcwq() was incorrectly triggering BUG_ON() if cpu number is
equal to or higher than num_possible_cpus() instead of nr_cpu_ids.
Fix it.
Signed-off-by: NTejun Heo <tj@kernel.org>

When one flusher is cascading to the next flusher, it first sets
wq->first_flusher to the next one and sets up the next flush cycle.
If there's nothing to do for the next cycle, it clears
wq->flush_flusher and proceeds to the one after that.

If the woken up flusher checks wq->first_flusher before it gets
cleared, it will incorrectly assume the role of the first flusher,
which triggers BUG_ON() sanity check.

Fix it by checking wq->first_flusher again after grabbing the mutex.
Signed-off-by: NTejun Heo <tj@kernel.org>

worker_set/clr_flags() assume that if none of NOT_RUNNING flags is set
the worker must be contributing to nr_running which is only true if
the worker is actually running.

As when called from self, it is guaranteed that the worker is running,
those functions can be safely used from the worker itself and they
aren't necessary from other places anyway.  Make the following changes
to fix the bug.

* Make worker_set/clr_flags() whine if not called from self.

* Convert all places which called those functions from other tasks to
  manipulate flags directly.

* Make trustee_thread() directly clear nr_running after setting
  WORKER_ROGUE on all workers.  This is the only place where
  nr_running manipulation is necessary outside of workers themselves.

* While at it, add sanity check for nr_running in worker_enter_idle().
Signed-off-by: NTejun Heo <tj@kernel.org>

This patch implements cpu intensive workqueue which can be specified
with WQ_CPU_INTENSIVE flag on creation.  Works queued to a cpu
intensive workqueue don't participate in concurrency management.  IOW,
it doesn't contribute to gcwq->nr_running and thus doesn't delay
excution of other works.

Note that although cpu intensive works won't delay other works, they
can be delayed by other works.  Combine with WQ_HIGHPRI to avoid being
delayed by other works too.

As the name suggests this is useful when using workqueue for cpu
intensive works.  Workers executing cpu intensive works are not
considered for workqueue concurrency management and left for the
scheduler to manage.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>

This patch implements high priority workqueue which can be specified
with WQ_HIGHPRI flag on creation.  A high priority workqueue has the
following properties.

* A work queued to it is queued at the head of the worklist of the
  respective gcwq after other highpri works, while normal works are
  always appended at the end.

* As long as there are highpri works on gcwq->worklist,
  [__]need_more_worker() remains %true and process_one_work() wakes up
  another worker before it start executing a work.

The above two properties guarantee that works queued to high priority
workqueues are dispatched to workers and start execution as soon as
possible regardless of the state of other works.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Andrew Morton <akpm@linux-foundation.org>

Implement the following utility APIs.

 workqueue_set_max_active()	: adjust max_active of a wq
 workqueue_congested()		: test whether a wq is contested
 work_cpu()			: determine the last / current cpu of a work
 work_busy()			: query whether a work is busy

* Anton Blanchard fixed missing ret initialization in work_busy().
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Anton Blanchard <anton@samba.org>

This patch makes changes to make new workqueue features available to
its users.

* Now that workqueue is more featureful, there should be a public
  workqueue creation function which takes paramters to control them.
  Rename __create_workqueue() to alloc_workqueue() and make 0
  max_active mean WQ_DFL_ACTIVE.  In the long run, all
  create_workqueue_*() will be converted over to alloc_workqueue().

* To further unify access interface, rename keventd_wq to system_wq
  and export it.

* Add system_long_wq and system_nrt_wq.  The former is to host long
  running works separately (so that flush_scheduled_work() dosen't
  take so long) and the latter guarantees any queued work item is
  never executed in parallel by multiple CPUs.  These will be used by
  future patches to update workqueue users.
Signed-off-by: NTejun Heo <tj@kernel.org>

Define WQ_MAX_ACTIVE and create keventd with max_active set to half of
it which means that keventd now can process upto WQ_MAX_ACTIVE / 2 - 1
works concurrently.  Unless some combination can result in dependency
loop longer than max_active, deadlock won't happen and thus it's
unnecessary to check whether current_is_keventd() before trying to
schedule a work.  Kill current_is_keventd().

(Lockdep annotations are broken.  We need lock_map_acquire_read_norecurse())
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Oleg Nesterov <oleg@redhat.com>

Instead of creating a worker for each cwq and putting it into the
shared pool, manage per-cpu workers dynamically.

Works aren't supposed to be cpu cycle hogs and maintaining just enough
concurrency to prevent work processing from stalling due to lack of
processing context is optimal.  gcwq keeps the number of concurrent
active workers to minimum but no less.  As long as there's one or more
running workers on the cpu, no new worker is scheduled so that works
can be processed in batch as much as possible but when the last
running worker blocks, gcwq immediately schedules new worker so that
the cpu doesn't sit idle while there are works to be processed.

gcwq always keeps at least single idle worker around.  When a new
worker is necessary and the worker is the last idle one, the worker
assumes the role of "manager" and manages the worker pool -
ie. creates another worker.  Forward-progress is guaranteed by having
dedicated rescue workers for workqueues which may be necessary while
creating a new worker.  When the manager is having problem creating a
new worker, mayday timer activates and rescue workers are summoned to
the cpu and execute works which might be necessary to create new
workers.

Trustee is expanded to serve the role of manager while a CPU is being
taken down and stays down.  As no new works are supposed to be queued
on a dead cpu, it just needs to drain all the existing ones.  Trustee
continues to try to create new workers and summon rescuers as long as
there are pending works.  If the CPU is brought back up while the
trustee is still trying to drain the gcwq from the previous offlining,
the trustee will kill all idles ones and tell workers which are still
busy to rebind to the cpu, and pass control over to gcwq which assumes
the manager role as necessary.

Concurrency managed worker pool reduces the number of workers
drastically.  Only workers which are necessary to keep the processing
going are created and kept.  Also, it reduces cache footprint by
avoiding unnecessarily switching contexts between different workers.

Please note that this patch does not increase max_active of any
workqueue.  All workqueues can still only process one work per cpu.
Signed-off-by: NTejun Heo <tj@kernel.org>

Implement worker_{set|clr}_flags() to manipulate worker flags.  These
are currently simple wrappers but logics to track the current worker
state and the current level of concurrency will be added.
Signed-off-by: NTejun Heo <tj@kernel.org>

Use gcwq->worklist instead of cwq->worklist and break the strict
association between a cwq and its worker.  All works queued on a cpu
are queued on gcwq->worklist and processed by any available worker on
the gcwq.

As there no longer is strict association between a cwq and its worker,
whether a work is executing can now only be determined by calling
[__]find_worker_executing_work().

After this change, the only association between a cwq and its worker
is that a cwq puts a worker into shared worker pool on creation and
kills it on destruction.  As all workqueues are still limited to
max_active of one, this means that there are always at least as many
workers as active works and thus there's no danger for deadlock.

The break of strong association between cwqs and workers requires
somewhat clumsy changes to current_is_keventd() and
destroy_workqueue().  Dynamic worker pool management will remove both
clumsy changes.  current_is_keventd() won't be necessary at all as the
only reason it exists is to avoid queueing a work from a work which
will be allowed just fine.  The clumsy part of destroy_workqueue() is
added because a worker can only be destroyed while idle and there's no
guarantee a worker is idle when its wq is going down.  With dynamic
pool management, workers are not associated with workqueues at all and
only idle ones will be submitted to destroy_workqueue() so the code
won't be necessary anymore.
Signed-off-by: NTejun Heo <tj@kernel.org>

With gcwq managing all the workers and work->data pointing to the last
gcwq it was on, non-reentrance can be easily implemented by checking
whether the work is still running on the previous gcwq on queueing.
Implement it.
Signed-off-by: NTejun Heo <tj@kernel.org>

To implement non-reentrant workqueue, the last gcwq a work was
executed on must be reliably obtainable as long as the work structure
is valid even if the previous workqueue has been destroyed.

To achieve this, work->data will be overloaded to carry the last cpu
number once execution starts so that the previous gcwq can be located
reliably.  This means that cwq can't be obtained from work after
execution starts but only gcwq.

Implement set_work_{cwq|cpu}(), get_work_[g]cwq() and
clear_work_data() to set work data to the cpu number when starting
execution, access the overloaded work data and clear it after
cancellation.

queue_delayed_work_on() is updated to preserve the last cpu while
in-flight in timer and other callers which depended on getting cwq
from work after execution starts are converted to depend on gcwq
instead.

* Anton Blanchard fixed compile error on powerpc due to missing
  linux/threads.h include.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Anton Blanchard <anton@samba.org>

Now that all the workers are tracked by gcwq, we can find which worker
is executing a work from gcwq.  Implement find_worker_executing_work()
and make worker track its current_cwq so that we can find things the
other way around.  This will be used to implement non-reentrant wqs.
Signed-off-by: NTejun Heo <tj@kernel.org>

Reimplement st (single thread) workqueue so that it's friendly to
shared worker pool.  It was originally implemented by confining st
workqueues to use cwq of a fixed cpu and always having a worker for
the cpu.  This implementation isn't very friendly to shared worker
pool and suboptimal in that it ends up crossing cpu boundaries often.

Reimplement st workqueue using dynamic single cpu binding and
cwq->limit.  WQ_SINGLE_THREAD is replaced with WQ_SINGLE_CPU.  In a
single cpu workqueue, at most single cwq is bound to the wq at any
given time.  Arbitration is done using atomic accesses to
wq->single_cpu when queueing a work.  Once bound, the binding stays
till the workqueue is drained.

Note that the binding is never broken while a workqueue is frozen.
This is because idle cwqs may have works waiting in delayed_works
queue while frozen.  On thaw, the cwq is restarted if there are any
delayed works or unbound otherwise.

When combined with max_active limit of 1, single cpu workqueue has
exactly the same execution properties as the original single thread
workqueue while allowing sharing of per-cpu workers.
Signed-off-by: NTejun Heo <tj@kernel.org>

Reimplement CPU hotplugging support using trustee thread.  On CPU
down, a trustee thread is created and each step of CPU down is
executed by the trustee and workqueue_cpu_callback() simply drives and
waits for trustee state transitions.

CPU down operation no longer waits for works to be drained but trustee
sticks around till all pending works have been completed.  If CPU is
brought back up while works are still draining,
workqueue_cpu_callback() tells trustee to step down and tell workers
to rebind to the cpu.

As it's difficult to tell whether cwqs are empty if it's freezing or
frozen, trustee doesn't consider draining to be complete while a gcwq
is freezing or frozen (tracked by new GCWQ_FREEZING flag).  Also,
workers which get unbound from their cpu are marked with WORKER_ROGUE.

Trustee based implementation doesn't bring any new feature at this
point but it will be used to manage worker pool when dynamic shared
worker pool is implemented.
Signed-off-by: NTejun Heo <tj@kernel.org>

Implement worker states.  After created, a worker is STARTED.  While a
worker isn't processing a work, it's IDLE and chained on
gcwq->idle_list.  While processing a work, a worker is BUSY and
chained on gcwq->busy_hash.  Also, gcwq now counts the number of all
workers and idle ones.

worker_thread() is restructured to reflect state transitions.
cwq->more_work is removed and waking up a worker makes it check for
events.  A worker is killed by setting DIE flag while it's IDLE and
waking it up.

This gives gcwq better visibility of what's going on and allows it to
find out whether a work is executing quickly which is necessary to
have multiple workers processing the same cwq.
Signed-off-by: NTejun Heo <tj@kernel.org>

There is one gcwq (global cwq) per each cpu and all cwqs on an cpu
point to it.  A gcwq contains a lock to be used by all cwqs on the cpu
and an ida to give IDs to workers belonging to the cpu.

This patch introduces gcwq, moves worker_ida into gcwq and make all
cwqs on the same cpu use the cpu's gcwq->lock instead of separate
locks.  gcwq->ida is now protected by gcwq->lock too.
Signed-off-by: NTejun Heo <tj@kernel.org>

Currently, workqueue freezing is implemented by marking the worker
freezeable and calling try_to_freeze() from dispatch loop.
Reimplement it using cwq->limit so that the workqueue is frozen
instead of the worker.

* workqueue_struct->saved_max_active is added which stores the
  specified max_active on initialization.

* On freeze, all cwq->max_active's are quenched to zero.  Freezing is
  complete when nr_active on all cwqs reach zero.

* On thaw, all cwq->max_active's are restored to wq->saved_max_active
  and the worklist is repopulated.

This new implementation allows having single shared pool of workers
per cpu.
Signed-off-by: NTejun Heo <tj@kernel.org>

Add cwq->nr_active, cwq->max_active and cwq->delayed_work.  nr_active
counts the number of active works per cwq.  A work is active if it's
flushable (colored) and is on cwq's worklist.  If nr_active reaches
max_active, new works are queued on cwq->delayed_work and activated
later as works on the cwq complete and decrement nr_active.

cwq->max_active can be specified via the new @max_active parameter to
__create_workqueue() and is set to 1 for all workqueues for now.  As
each cwq has only single worker now, this double queueing doesn't
cause any behavior difference visible to its users.

This will be used to reimplement freeze/thaw and implement shared
worker pool.
Signed-off-by: NTejun Heo <tj@kernel.org>

A work is linked to the next one by having WORK_STRUCT_LINKED bit set
and these links can be chained.  When a linked work is dispatched to a
worker, all linked works are dispatched to the worker's newly added
->scheduled queue and processed back-to-back.

Currently, as there's only single worker per cwq, having linked works
doesn't make any visible behavior difference.  This change is to
prepare for multiple shared workers per cpu.
Signed-off-by: NTejun Heo <tj@kernel.org>

Separate out worker thread related information to struct worker from
struct cpu_workqueue_struct and implement helper functions to deal
with the new struct worker.  The only change which is visible outside
is that now workqueue worker are all named "kworker/CPUID:WORKERID"
where WORKERID is allocated from per-cpu ida.

This is in preparation of concurrency managed workqueue where shared
multiple workers would be available per cpu.
Signed-off-by: NTejun Heo <tj@kernel.org>

Reimplement workqueue flushing using color coded works.  wq has the
current work color which is painted on the works being issued via
cwqs.  Flushing a workqueue is achieved by advancing the current work
colors of cwqs and waiting for all the works which have any of the
previous colors to drain.

Currently there are 16 possible colors, one is reserved for no color
and 15 colors are useable allowing 14 concurrent flushes.  When color
space gets full, flush attempts are batched up and processed together
when color frees up, so even with many concurrent flushers, the new
implementation won't build up huge queue of flushers which has to be
processed one after another.

Only works which are queued via __queue_work() are colored.  Works
which are directly put on queue using insert_work() use NO_COLOR and
don't participate in workqueue flushing.  Currently only works used
for work-specific flush fall in this category.

This new implementation leaves only cleanup_workqueue_thread() as the
user of flush_cpu_workqueue().  Just make its users use
flush_workqueue() and kthread_stop() directly and kill
cleanup_workqueue_thread().  As workqueue flushing doesn't use barrier
request anymore, the comment describing the complex synchronization
around it in cleanup_workqueue_thread() is removed together with the
function.

This new implementation is to allow having and sharing multiple
workers per cpu.

Please note that one more bit is reserved for a future work flag by
this patch.  This is to avoid shifting bits and updating comments
later.
Signed-off-by: NTejun Heo <tj@kernel.org>