[PATCH] scsi: ufs: Fix a possible dead lock in clock scaling

[Can Guo <cang@xxxxxxxxxxxxxx>]

Assume a scenario where task A and B call ufshcd_devfreq_scale()
simultaneously. After task B calls downgrade_write() [1], but before it
calls down_read() [3], if task A calls down_write() [2], when task B calls
down_read() [3], it will lead to dead lock. Fix this by utilizing the
existing flag scaling.is_allowed to make sure only one task can do clock
scaling at a time.

Task A -
down_write [2]
ufshcd_clock_scaling_prepare
ufshcd_devfreq_scale
ufshcd_clkscale_enable_store

Task B -
down_read [3]
ufshcd_exec_dev_cmd
ufshcd_query_flag
ufshcd_wb_ctrl
downgrade_write [1]
ufshcd_devfreq_scale
ufshcd_devfreq_target
devfreq_set_target
update_devfreq
devfreq_performance_handler
governor_store

Fixes: 0e9d4ca43ba81 ("scsi: ufs: Protect some contexts from unexpected clock scaling")
Signed-off-by: Can Guo <cang@xxxxxxxxxxxxxx>

diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index 3841ab49..782a9c8 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c
@@ -1186,6 +1186,7 @@ static int ufshcd_clock_scaling_prepare(struct ufs_hba *hba)
 		goto out;
 	}
 
+	hba->clk_scaling.is_allowed = false;
 	/* let's not get into low power until clock scaling is completed */
 	ufshcd_hold(hba, false);
 
@@ -1193,12 +1194,10 @@ static int ufshcd_clock_scaling_prepare(struct ufs_hba *hba)
 	return ret;
 }
 
-static void ufshcd_clock_scaling_unprepare(struct ufs_hba *hba, bool writelock)
+static void ufshcd_clock_scaling_unprepare(struct ufs_hba *hba)
 {
-	if (writelock)
-		up_write(&hba->clk_scaling_lock);
-	else
-		up_read(&hba->clk_scaling_lock);
+	hba->clk_scaling.is_allowed = true;
+	up_write(&hba->clk_scaling_lock);
 	ufshcd_scsi_unblock_requests(hba);
 	ufshcd_release(hba);
 }
@@ -1215,7 +1214,6 @@ static void ufshcd_clock_scaling_unprepare(struct ufs_hba *hba, bool writelock)
 static int ufshcd_devfreq_scale(struct ufs_hba *hba, bool scale_up)
 {
 	int ret = 0;
-	bool is_writelock = true;
 
 	ret = ufshcd_clock_scaling_prepare(hba);
 	if (ret)
@@ -1245,12 +1243,12 @@ static int ufshcd_devfreq_scale(struct ufs_hba *hba, bool scale_up)
 	}
 
 	/* Enable Write Booster if we have scaled up else disable it */
-	downgrade_write(&hba->clk_scaling_lock);
-	is_writelock = false;
+	up_write(&hba->clk_scaling_lock);
 	ufshcd_wb_toggle(hba, scale_up);
+	down_write(&hba->clk_scaling_lock);
 
 out_unprepare:
-	ufshcd_clock_scaling_unprepare(hba, is_writelock);
+	ufshcd_clock_scaling_unprepare(hba);
 	return ret;
 }
 
-- 
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