Some background.  The preemption was disabled before to guarantee that a
preloaded object is available for a CPU, it was stored for.  That was
achieved by combining the disabling the preemption and taking the spin
lock while the ne_fit_preload_node is checked.

The aim was to not allocate in atomic context when spinlock is taken
later, for regular vmap allocations.  But that approach conflicts with
CONFIG_PREEMPT_RT philosophy.  It means that calling spin_lock() with
disabled preemption is forbidden in the CONFIG_PREEMPT_RT kernel.

Therefore, get rid of preempt_disable() and preempt_enable() when the
preload is done for splitting purpose.  As a result we do not guarantee
now that a CPU is preloaded, instead we minimize the case when it is
not, with this change, by populating the per cpu preload pointer under
the vmap_area_lock.

This implies that at least each caller that has done the preallocation
will not fallback to an atomic allocation later.  It is possible that
the preallocation would be pointless or that no preallocation is done
because of the race but the data shows that this is really rare.

For example i run the special test case that follows the preload pattern
and path.  20 "unbind" threads run it and each does 1000000 allocations.
Only 3.5 times among 1000000 a CPU was not preloaded.  So it can happen
but the number is negligible.

[mhocko@suse.com: changelog additions]
Link: http://lkml.kernel.org/r/20191016095438.12391-1-urezki@gmail.com
Fixes: 82dd23e8

 ("mm/vmalloc.c: preload a CPU with one object for split purpose")
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Daniel Wagner <dwagner@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>