[PATCH 2/2] scsi: ufs: Fix a deadlock in the error handler

[Bart Van Assche <bvanassche@acm.org>]

The following deadlock has been observed on a test setup:
* All tags allocated.
* The SCSI error handler calls ufshcd_eh_host_reset_handler()
* ufshcd_eh_host_reset_handler() queues work that calls ufshcd_err_handler()
* ufshcd_err_handler() locks up as follows:

Workqueue: ufs_eh_wq_0 ufshcd_err_handler.cfi_jt
Call trace:
 __switch_to+0x298/0x5d8
 __schedule+0x6cc/0xa94
 schedule+0x12c/0x298
 blk_mq_get_tag+0x210/0x480
 __blk_mq_alloc_request+0x1c8/0x284
 blk_get_request+0x74/0x134
 ufshcd_exec_dev_cmd+0x68/0x640
 ufshcd_verify_dev_init+0x68/0x35c
 ufshcd_probe_hba+0x12c/0x1cb8
 ufshcd_host_reset_and_restore+0x88/0x254
 ufshcd_reset_and_restore+0xd0/0x354
 ufshcd_err_handler+0x408/0xc58
 process_one_work+0x24c/0x66c
 worker_thread+0x3e8/0xa4c
 kthread+0x150/0x1b4
 ret_from_fork+0x10/0x30

Fix this lockup by making ufshcd_exec_dev_cmd() allocate a reserved
request.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/scsi/ufs/ufshcd.c | 20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index 9d964b979aa2..6b0101169974 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c
@@ -2904,12 +2904,7 @@ static int ufshcd_exec_dev_cmd(struct ufs_hba *hba,
 
 	down_read(&hba->clk_scaling_lock);
 
-	/*
-	 * Get free slot, sleep if slots are unavailable.
-	 * Even though we use wait_event() which sleeps indefinitely,
-	 * the maximum wait time is bounded by SCSI request timeout.
-	 */
-	req = blk_get_request(q, REQ_OP_DRV_OUT, 0);
+	req = blk_mq_alloc_request(q, REQ_OP_DRV_OUT, BLK_MQ_REQ_RESERVED);
 	if (IS_ERR(req)) {
 		err = PTR_ERR(req);
 		goto out_unlock;
@@ -4919,11 +4914,7 @@ static int ufshcd_slave_alloc(struct scsi_device *sdev)
  */
 static int ufshcd_change_queue_depth(struct scsi_device *sdev, int depth)
 {
-	struct ufs_hba *hba = shost_priv(sdev->host);
-
-	if (depth > hba->nutrs)
-		depth = hba->nutrs;
-	return scsi_change_queue_depth(sdev, depth);
+	return scsi_change_queue_depth(sdev, min(depth, sdev->host->can_queue));
 }
 
 static void ufshcd_hpb_destroy(struct ufs_hba *hba, struct scsi_device *sdev)
@@ -8124,7 +8115,8 @@ static struct scsi_host_template ufshcd_driver_template = {
 	.this_id		= -1,
 	.sg_tablesize		= SG_ALL,
 	.cmd_per_lun		= UFSHCD_CMD_PER_LUN,
-	.can_queue		= UFSHCD_CAN_QUEUE,
+	.can_queue		= UFSHCD_CAN_QUEUE - 1,
+	.reserved_tags		= 1,
 	.max_segment_size	= PRDT_DATA_BYTE_COUNT_MAX,
 	.max_host_blocked	= 1,
 	.track_queue_depth	= 1,
@@ -9485,8 +9477,8 @@ int ufshcd_init(struct ufs_hba *hba, void __iomem *mmio_base, unsigned int irq)
 	/* Configure LRB */
 	ufshcd_host_memory_configure(hba);
 
-	host->can_queue = hba->nutrs;
-	host->cmd_per_lun = hba->nutrs;
+	host->can_queue = hba->nutrs - 1;
+	host->cmd_per_lun = hba->nutrs - 1;
 	host->max_id = UFSHCD_MAX_ID;
 	host->max_lun = UFS_MAX_LUNS;
 	host->max_channel = UFSHCD_MAX_CHANNEL;