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>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8b309e81ed0fda22812787c93b23a2d568f1f966..06ab0683a1f16c716ec4c4e19bcb64061cb6ab27 100644 (file)
--- a/block/blk-mq.c
+++ b/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)