scsi: ufs: Add clock ungating to a separate workqueue

UFS driver can receive a request during memory reclaim by kswapd.  So
when ufs driver puts the ungate work in queue, and if there are no idle
workers, kthreadd is invoked to create a new kworker. Since kswapd task
holds a mutex which kthreadd also needs, this can cause a deadlock
situation. So ungate work must be done in a separate work queue with
WQ_MEM_RECLAIM flag enabled.  Such a workqueue will have a rescue thread
which will be called when the above deadlock condition is possible.

Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>
Signed-off-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/ufs/ufshcd.c

@@ -1532,7 +1532,8 @@ start:
 		hba->clk_gating.state = REQ_CLKS_ON;
 		trace_ufshcd_clk_gating(dev_name(hba->dev),
 					hba->clk_gating.state);
-		schedule_work(&hba->clk_gating.ungate_work);
+		queue_work(hba->clk_gating.clk_gating_workq,
+			   &hba->clk_gating.ungate_work);
 		/*
 		 * fall through to check if we should wait for this
 		 * work to be done or not.
@@ -1718,6 +1719,8 @@ out:
 
 static void ufshcd_init_clk_gating(struct ufs_hba *hba)
 {
+	char wq_name[sizeof("ufs_clk_gating_00")];
+
 	if (!ufshcd_is_clkgating_allowed(hba))
 		return;
 
@@ -1725,6 +1728,11 @@ static void ufshcd_init_clk_gating(struct ufs_hba *hba)
 	INIT_DELAYED_WORK(&hba->clk_gating.gate_work, ufshcd_gate_work);
 	INIT_WORK(&hba->clk_gating.ungate_work, ufshcd_ungate_work);
 
+	snprintf(wq_name, ARRAY_SIZE(wq_name), "ufs_clk_gating_%d",
+		 hba->host->host_no);
+	hba->clk_gating.clk_gating_workq = alloc_ordered_workqueue(wq_name,
+							   WQ_MEM_RECLAIM);
+
 	hba->clk_gating.is_enabled = true;
 
 	hba->clk_gating.delay_attr.show = ufshcd_clkgate_delay_show;
@@ -1752,6 +1760,7 @@ static void ufshcd_exit_clk_gating(struct ufs_hba *hba)
 	device_remove_file(hba->dev, &hba->clk_gating.enable_attr);
 	cancel_work_sync(&hba->clk_gating.ungate_work);
 	cancel_delayed_work_sync(&hba->clk_gating.gate_work);
+	destroy_workqueue(hba->clk_gating.clk_gating_workq);
 }
 
 /* Must be called with host lock acquired */

drivers/scsi/ufs/ufshcd.h

@@ -362,6 +362,7 @@ struct ufs_clk_gating {
 	struct device_attribute enable_attr;
 	bool is_enabled;
 	int active_reqs;
+	struct workqueue_struct *clk_gating_workq;
 };
 
 struct ufs_saved_pwr_info {