[PATCH] scsi: ufs: Fix deadlocks between power management and error handler

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

The following deadlocks have been observed on multiple test setups:

* ufshcd_wl_suspend() is waiting for blk_execute_rq() to complete while it
  holds host_sem.
* ufshcd_eh_host_reset_handler() invokes ufshcd_err_handler() and the
  latter function tries to obtain host_sem.
This is a deadlock because blk_execute_rq() can't execute SCSI commands
while the host is in the SHOST_RECOVERY state and because the error
handler cannot make progress either.

* ufshcd_wl_runtime_resume() is waiting for blk_execute_rq() to finish
  while it holds host_sem.
* ufshcd_eh_host_reset_handler() invokes ufshcd_err_handler() and the
  latter function calls pm_runtime_resume().
This is a deadlock because of the same reason as the previous scenario.

Fix both deadlocks by not obtaining host_sem from the power management
code paths. Removing the host_sem locking from the power management code
is safe because the ufshcd_err_handler() is already serialized against
SCSI command execution.

The ufshcd_rpm_get_sync() call at the start of
ufshcd_err_handling_prepare() may deadlock since calling scsi_execute()
is required by the UFS runtime resume implementation. Fixing that
deadlock falls outside the scope of this patch.

Cc: dh0421.hwang@samsung.com
Cc: Asutosh Das <asutoshd@codeaurora.org>
Fixes: b294ff3e3449 ("scsi: ufs: core: Enable power management for wlun")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/ufs/core/ufshcd.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index 7c15cbc737b4..cd3c2aa981c6 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -9254,16 +9254,13 @@ static int ufshcd_wl_suspend(struct device *dev)
 	ktime_t start = ktime_get();
 
 	hba = shost_priv(sdev->host);
-	down(&hba->host_sem);
 
 	if (pm_runtime_suspended(dev))
 		goto out;
 
 	ret = __ufshcd_wl_suspend(hba, UFS_SYSTEM_PM);
-	if (ret) {
+	if (ret)
 		dev_err(&sdev->sdev_gendev, "%s failed: %d\n", __func__,  ret);
-		up(&hba->host_sem);
-	}
 
 out:
 	if (!ret)
@@ -9296,7 +9293,6 @@ static int ufshcd_wl_resume(struct device *dev)
 		hba->curr_dev_pwr_mode, hba->uic_link_state);
 	if (!ret)
 		hba->is_sys_suspended = false;
-	up(&hba->host_sem);
 	return ret;
 }
 #endif