percpu_ref: allow operation mode switching operations to be called concurrently

percpu_ref initially didn't have explicit mode switching operations.
It started out in percpu mode and switched to atomic mode on kill and
then released.  Ensuring that kill operation is initiated only after
init completes was naturally the caller's responsibility.

percpu_ref_reinit() was introduced later but it didn't shift the
synchronization responsibility.  Reinit can't be performed until kill
is confirmed, so there was nothing to worry about
synchronization-wise.  Also, as both reinit and kill manipulate the
base reference, invocations of the same function couldn't be allowed
to race each other.

The latest additions of percpu_ref_switch_to_atomic/percpu() changed
the situation.  These two functions can be called any time as long as
the percpu_ref is between init and exit and thus there are valid valid
usage scenarios where these new functions race with each other or
against reinit/kill.  Mostly from inertia, f47ad457 ("percpu_ref:
decouple switching to percpu mode and reinit") still left
synchronization among percpu mode switching operations to its users.

That the new switch functions can be freely mixed with kill/reinit but
the operations themselves should be synchronized is too subtle a
requirement and led to a very subtle race condition in blk-mq freezing
path.

This patch fixes the situation by introducing percpu_ref_switch_lock
to protect mode switching operations.  This ensures that percpu-ref
users don't have to worry about mode changing operations racing
against each other, e.g. switch_to_percpu against kill, as long as the
sequence of operations is valid.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Akinobu Mita <akinobu.mita@gmail.com>
Link: http://lkml.kernel.org/g/1443287365-4244-7-git-send-email-akinobu.mita@gmail.com
Fixes: f47ad457 ("percpu_ref: decouple switching to percpu mode and reinit")