[PATCH 1/2] blk-mq: make sure hctx->next_cpu is set correctly

Ming Lei
2018-01-17T12:35:09Z

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 is triggered 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.

This patch deals with this issue by re-selecting next CPU, and make
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>
---
block/blk-mq.c | 31 +++++++++++++++++++++++++++++--
1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c376d1b6309a..dc4066d28323 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1416,21 +1416,48 @@ 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 here, and this may happen when
+ * running from blk_mq_hctx_notify_dead(), and 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;
}

--
2.9.5