[PATCH] blk-mq: avoid to hang in the cpuhp offline handler

[Ming Lei]

For avoiding to trigger io timeout when one hctx becomes inactive, we
drain IOs when all CPUs of one hctx are offline. However, driver's
timeout handler may require cpus_read_lock, such as nvme-pci,
pci_alloc_irq_vectors_affinity() is called in nvme-pci reset context,
and irq_build_affinity_masks() needs cpus_read_lock().

Meantime when blk-mq's cpuhp offline handler is called, cpus_write_lock
is held, so deadlock is caused.

Fixes the issue by breaking the wait loop if enough long time elapses,
and these in-flight not drained IO still can be handled by timeout
handler.

Cc: linux-nvme at lists.infradead.org
Reported-by: Yi Zhang <yi.zhang at redhat.com>
Fixes: bf0beec0607d ("blk-mq: drain I/O when all CPUs in a hctx are offline")
Signed-off-by: Ming Lei <ming.lei at redhat.com>
---
 block/blk-mq.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c96c8c4f751b..4585985b8537 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3301,6 +3301,7 @@ static inline bool blk_mq_last_cpu_in_hctx(unsigned int cpu,
 	return true;
 }
 
+#define BLK_MQ_MAX_OFFLINE_WAIT_MSECS 3000
 static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
 {
 	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
@@ -3326,8 +3327,13 @@ static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
 	 * frozen and there are no requests.
 	 */
 	if (percpu_ref_tryget(&hctx->queue->q_usage_counter)) {
-		while (blk_mq_hctx_has_requests(hctx))
+		unsigned int wait_ms = 0;
+
+		while (blk_mq_hctx_has_requests(hctx) && wait_ms <
+				BLK_MQ_MAX_OFFLINE_WAIT_MSECS) {
 			msleep(5);
+			wait_ms += 5;
+		}
 		percpu_ref_put(&hctx->queue->q_usage_counter);
 	}
 
-- 
2.31.1