blk-mq: make sure hctx->next_cpu is set correctly

When hctx->next_cpu is set from possible online CPUs, there is one
race in which hctx->next_cpu may be set as >= nr_cpu_ids, and finally
break workqueue.

The race can be triggered in the following two sitations:

1) when one CPU is becoming DEAD, blk_mq_hctx_notify_dead() is called
to dispatch requests from the DEAD cpu context, but at that
time, this DEAD CPU has been cleared from 'cpu_online_mask', so all
CPUs in hctx->cpumask may become offline, and cause hctx->next_cpu set
a bad value.

2) blk_mq_delay_run_hw_queue() is called from CPU B, and found the queue
should be run on the other CPU A, then CPU A may become offline at the
same time and all CPUs in hctx->cpumask become offline.

This patch deals with this issue by re-selecting next CPU, and making
sure it is set correctly.

Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Stefan Haberland <sth@linux.vnet.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-by: "jianchao.wang" <jianchao.w.wang@oracle.com>
Tested-by: "jianchao.wang" <jianchao.w.wang@oracle.com>
Fixes: 20e4d81393 ("blk-mq: simplify queue mapping & schedule with each possisble CPU")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c8f62e6be6b6acf183e86ffe2594be0c9076c2d4..3bd41f1066ee13db678c9b7b53300f576301c1cc 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1319,21 +1319,47 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
  */
 static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
 {
+       bool tried = false;
+
        if (hctx->queue->nr_hw_queues == 1)
                return WORK_CPU_UNBOUND;
 
        if (--hctx->next_cpu_batch <= 0) {
                int next_cpu;
-
+select_cpu:
                next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
                                cpu_online_mask);
                if (next_cpu >= nr_cpu_ids)
                        next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);
 
-               hctx->next_cpu = next_cpu;
+               /*
+                * No online CPU is found, so have to make sure hctx->next_cpu
+                * is set correctly for not breaking workqueue.
+                */
+               if (next_cpu >= nr_cpu_ids)
+                       hctx->next_cpu = cpumask_first(hctx->cpumask);
+               else
+                       hctx->next_cpu = next_cpu;
                hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
        }
 
+       /*
+        * Do unbound schedule if we can't find a online CPU for this hctx,
+        * and it should only happen in the path of handling CPU DEAD.
+        */
+       if (!cpu_online(hctx->next_cpu)) {
+               if (!tried) {
+                       tried = true;
+                       goto select_cpu;
+               }
+
+               /*
+                * Make sure to re-select CPU next time once after CPUs
+                * in hctx->cpumask become online again.
+                */
+               hctx->next_cpu_batch = 1;
+               return WORK_CPU_UNBOUND;
+       }
        return hctx->next_cpu;
 }