On 8/9/21 7:13 AM, lijinlin3@xxxxxxxxxx wrote:
From: Li Jinlin <lijinlin3@xxxxxxxxxx>
We found a hang issue, the test steps are as follows:
1. blocking device via scsi_device_set_state()
2. dd if=/dev/sda of=/mnt/t.log bs=1M count=10
3. echo none > /sys/block/sda/queue/scheduler
4. echo "running" >/sys/block/sda/device/state
Step 3 and 4 should finish this work after step 4, but they hangs.
CPU#0 CPU#1 CPU#2
--------------- ---------------- ----------------
Step 1: blocking device
Step 2: dd xxxx
^^^^^^ get request
q_usage_counter++
Step 3: switching scheculer
elv_iosched_store
elevator_switch
blk_mq_freeze_queue
blk_freeze_queue
> blk_freeze_queue_start
^^^^^^ mq_freeze_depth++
> blk_mq_run_hw_queues
^^^^^^ can't run queue when dev blocked
> blk_mq_freeze_queue_wait
^^^^^^ Hang here!!!
wait q_usage_counter==0
Step 4: running device
store_state_field
scsi_rescan_device
scsi_attach_vpd
scsi_vpd_inquiry
__scsi_execute
blk_get_request
blk_mq_alloc_request
blk_queue_enter
^^^^^^ Hang here!!!
wait mq_freeze_depth==0
blk_mq_run_hw_queues
^^^^^^ dispatch IO, q_usage_counter will reduce to zero
blk_mq_unfreeze_queue
^^^^^ mq_freeze_depth--
Step 3 and 4 wait for each other.
To fix this, we need to run queue before rescanning device when the device
state changes to SDEV_RUNNING.
Fixes: f0f82e2476f6 ("scsi: core: Fix capacity set to zero after offlinining device")
Signed-off-by: Li Jinlin <lijinlin3@xxxxxxxxxx>
Signed-off-by: Qiu Laibin <qiulaibin@xxxxxxxxxx>
---
changes since v1 send with Message-ID:
20210805143231.1713299-1-lijinlin3@xxxxxxxxxx
- Modify the subject to make it distinct
- Modify the message to fix typo and make it distinct
- Reduce the number of SOB
drivers/scsi/scsi_sysfs.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index c3a710bceba0..aa701582c950 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -809,12 +809,12 @@ store_state_field(struct device *dev, struct device_attribute *attr,
ret = scsi_device_set_state(sdev, state);
/*
* If the device state changes to SDEV_RUNNING, we need to
- * rescan the device to revalidate it, and run the queue to
- * avoid I/O hang.
+ * run the queue to avoid I/O hang, and rescan the device
+ * to revalidate it.
*/
if (ret == 0 && state == SDEV_RUNNING) {
- scsi_rescan_device(dev);
blk_mq_run_hw_queues(sdev->request_queue, true);
+ scsi_rescan_device(dev);
}
mutex_unlock(&sdev->state_mutex);
The patch looks fine to me but I think the comment in
store_state_field() should be expanded. Although the description in the
commit message makes it clear how I/O may hang, that is not clear from
the source code comment. Please mention in the comment that running the
queue first is necessary because another thread may be waiting inside
blk_mq_freeze_queue_wait() and because that call may be waiting for
pending I/O to finish.
Thanks,
Bart.
