mm: slub: move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context
flush_all() flushes a specific SLAB cache on each CPU (where the cache
is present). The deactivate_slab()/__free_slab() invocation happens
within IPI handler and is problematic for PREEMPT_RT.
The flush operation is not a frequent operation or a hot path. The
per-CPU flush operation can be moved to within a workqueue.
Because a workqueue handler, unlike IPI handler, does not disable irqs,
flush_slab() now has to disable them for working with the kmem_cache_cpu
fields. deactivate_slab() is safe to call with irqs enabled.
[vbabka@suse.cz: adapt to new SLUB changes] Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>