NG
or distinguishing the two different cases.

While the current code is technically correct, not distinguishing the
two cases makes it difficult to use try_to_grab_pending() for other
purposes than canceling because it's impossible to tell whether it's
safe to busy-retry grabbing.

This patch adds a mechanism to mark a work item being canceled.
try_to_grab_pending() now disables irq on success and returns -EAGAIN
to indicate that grabbing failed but PENDING and queued states are
gonna agree soon and it's safe to busy-loop.  It returns -ENOENT if
the work item is being canceled and it may stay PENDING && !queued for
arbitrary amount of time.

__cancel_work_timer() is modified to mark the work canceling with
WORK_OFFQ_CANCELING after grabbing PENDING, thus making
try_to_grab_pending() fail with -ENOENT instead of -EAGAIN.  Also, it
invokes wait_on_work() iff grabbing failed with -ENOENT.  This isn't
necessary for correctness but makes it consistent with other future
users of try_to_grab_pending().

v2: try_to_grab_pending() was testing preempt_count() to ensure that
    the caller has disabled preemption.  This triggers spuriously if
    !CONFIG_PREEMPT_COUNT.  Use preemptible() instead.  Reported by
    Fengguang Wu.

v3: Updated so that try_to_grab_pending() disables irq on success
    rather than requiring preemption disabled by the caller.  This
    makes busy-looping easier and will allow try_to_grap_pending() to
    be used from bh/irq contexts.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Fengguang Wu <fengguang.wu@intel.com>
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