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 make
sure it is set correctly.
Cc: Christian Borntraeger <borntrae...@de.ibm.com>
Cc: Stefan Haberland <s...@linux.vnet.ibm.com>
Cc: Christoph Hellwig <h...@lst.de>
Cc: Thomas Gleixner <t...@linutronix.de>
Reported-by: "jianchao.wang" <jianchao.w.w...@oracle.com>
Tested-by: "jianchao.wang" <jianchao.w.w...@oracle.com>
Fixes: 20e4d81393 ("blk-mq: simplify queue mapping & schedule with each
possisble CPU")
Signed-off-by: Ming Lei <ming....@redhat.com>
---
block/blk-mq.c | 30 ++++++++++++++++++++++++++++--
1 file changed, 28 insertions(+), 2 deletions(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index c376d1b6309a..69e73b4e32f3 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1416,21 +1416,47 @@ 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;
}
--
2.9.5
