blk-mq: use get_cpu/put_cpu instead of preempt_disable/preempt_enable

blk-mq is using preempt_disable/enable in order to ensure that the
queue runners are placed on the right CPU.  This does not work with
the RT patches, because __blk_mq_run_hw_queue takes a non-raw
spinlock with the preemption-disabled region.  If there is contention
on the lock, this violates the rules for preemption-disabled regions.

While this should be easily fixable within the RT patches just by doing
migrate_disable/enable, we can do better and document _why_ this
particular region runs with disabled preemption.  After the previous
patch, it is trivial to switch it to get/put_cpu; the RT patches then
can change it to get_cpu_light, which lets virtio-blk run under RT
kernels.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Clark Williams <williams@redhat.com>
Tested-by: Clark Williams <williams@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -802,14 +802,14 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 		return;
 
 	if (!async) {
-		preempt_disable();
-		if (cpumask_test_cpu(smp_processor_id(), hctx->cpumask)) {
+		int cpu = get_cpu();
+		if (cpumask_test_cpu(cpu, hctx->cpumask)) {
 			__blk_mq_run_hw_queue(hctx);
-			preempt_enable();
+			put_cpu();
 			return;
 		}
 
-		preempt_enable();
+		put_cpu();
 	}
 
 	if (hctx->queue->nr_hw_queues == 1)