scsi: ufs: core: Fix devfreq deadlocks

There is a lock inversion and rwsem read-lock recursion in the devfreq
target callback which can lead to deadlocks.

Specifically, ufshcd_devfreq_scale() already holds a clk_scaling_lock
read lock when toggling the write booster, which involves taking the
dev_cmd mutex before taking another clk_scaling_lock read lock.

This can lead to a deadlock if another thread:

1) tries to acquire the dev_cmd and clk_scaling locks in the correct
order, or

2) takes a clk_scaling write lock before the attempt to take the
clk_scaling read lock a second time.

Fix this by dropping the clk_scaling_lock before toggling the write booster
as was done before commit 0e9d4ca43ba8 ("scsi: ufs: Protect some contexts
from unexpected clock scaling").

While the devfreq callbacks are already serialised, add a second
serialising mutex to handle the unlikely case where a callback triggered
through the devfreq sysfs interface is racing with a request to disable
clock scaling through the UFS controller 'clkscale_enable' sysfs
attribute. This could otherwise lead to the write booster being left
disabled after having disabled clock scaling.

Also take the new mutex in ufshcd_clk_scaling_allow() to make sure that any
pending write booster update has completed on return.

Note that this currently only affects Qualcomm platforms since commit
87bd05016a64 ("scsi: ufs: core: Allow host driver to disable wb toggling
during clock scaling").

The lock inversion (i.e. 1 above) was reported by lockdep as:

======================================================
WARNING: possible circular locking dependency detected
6.1.0-next-20221216 #211 Not tainted
------------------------------------------------------
kworker/u16:2/71 is trying to acquire lock:
ffff076280ba98a0 (&hba->dev_cmd.lock){+.+.}-{3:3}, at: ufshcd_query_flag+0x50/0x1c0

but task is already holding lock:
ffff076280ba9cf0 (&hba->clk_scaling_lock){++++}-{3:3}, at: ufshcd_devfreq_scale+0x2b8/0x380

which lock already depends on the new lock.
[ +0.011606]
the existing dependency chain (in reverse order) is:

-> #1 (&hba->clk_scaling_lock){++++}-{3:3}:
lock_acquire+0x68/0x90
down_read+0x58/0x80
ufshcd_exec_dev_cmd+0x70/0x2c0
ufshcd_verify_dev_init+0x68/0x170
ufshcd_probe_hba+0x398/0x1180
ufshcd_async_scan+0x30/0x320
async_run_entry_fn+0x34/0x150
process_one_work+0x288/0x6c0
worker_thread+0x74/0x450
kthread+0x118/0x120
ret_from_fork+0x10/0x20

-> #0 (&hba->dev_cmd.lock){+.+.}-{3:3}:
__lock_acquire+0x12a0/0x2240
lock_acquire.part.0+0xcc/0x220
lock_acquire+0x68/0x90
__mutex_lock+0x98/0x430
mutex_lock_nested+0x2c/0x40
ufshcd_query_flag+0x50/0x1c0
ufshcd_query_flag_retry+0x64/0x100
ufshcd_wb_toggle+0x5c/0x120
ufshcd_devfreq_scale+0x2c4/0x380
ufshcd_devfreq_target+0xf4/0x230
devfreq_set_target+0x84/0x2f0
devfreq_update_target+0xc4/0xf0
devfreq_monitor+0x38/0x1f0
process_one_work+0x288/0x6c0
worker_thread+0x74/0x450
kthread+0x118/0x120
ret_from_fork+0x10/0x20

other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&hba->clk_scaling_lock);
lock(&hba->dev_cmd.lock);
lock(&hba->clk_scaling_lock);
lock(&hba->dev_cmd.lock);

*** DEADLOCK ***

Fixes: 0e9d4ca43ba8 ("scsi: ufs: Protect some contexts from unexpected clock scaling")
Cc: [e-mail] # 5.12
Cc: Can Guo <[e-mail]>
Tested-by: Andrew Halaney <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>
Reviewed-by: Bart Van Assche <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/ufs/core/ufshcd.c

···
 	 * clock scaling is in progress
 	 */
 	ufshcd_scsi_block_requests(hba);
+	mutex_lock(&hba->wb_mutex);
 	down_write(&hba->clk_scaling_lock);
 
 	if (!hba->clk_scaling.is_allowed ||
 	    ufshcd_wait_for_doorbell_clr(hba, DOORBELL_CLR_TOUT_US)) {
 		ret = -EBUSY;
 		up_write(&hba->clk_scaling_lock);
+		mutex_unlock(&hba->wb_mutex);
 		ufshcd_scsi_unblock_requests(hba);
 		goto out;
 	}
···
 	return ret;
 }
 
-static void ufshcd_clock_scaling_unprepare(struct ufs_hba *hba, bool writelock)
+static void ufshcd_clock_scaling_unprepare(struct ufs_hba *hba, int err, bool scale_up)
 {
-	if (writelock)
-		up_write(&hba->clk_scaling_lock);
-	else
-		up_read(&hba->clk_scaling_lock);
+	up_write(&hba->clk_scaling_lock);
+
+	/* Enable Write Booster if we have scaled up else disable it */
+	if (ufshcd_enable_wb_if_scaling_up(hba) && !err)
+		ufshcd_wb_toggle(hba, scale_up);
+
+	mutex_unlock(&hba->wb_mutex);
+
 	ufshcd_scsi_unblock_requests(hba);
 	ufshcd_release(hba);
 }
···
 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)
···
 		}
 	}
 
-	/* Enable Write Booster if we have scaled up else disable it */
-	if (ufshcd_enable_wb_if_scaling_up(hba)) {
-		downgrade_write(&hba->clk_scaling_lock);
-		is_writelock = false;
-		ufshcd_wb_toggle(hba, scale_up);
-	}
-
 out_unprepare:
-	ufshcd_clock_scaling_unprepare(hba, is_writelock);
+	ufshcd_clock_scaling_unprepare(hba, ret, scale_up);
 	return ret;
 }
 
···
 
 static void ufshcd_clk_scaling_allow(struct ufs_hba *hba, bool allow)
 {
+	mutex_lock(&hba->wb_mutex);
 	down_write(&hba->clk_scaling_lock);
 	hba->clk_scaling.is_allowed = allow;
 	up_write(&hba->clk_scaling_lock);
+	mutex_unlock(&hba->wb_mutex);
 }
 
 static void ufshcd_clk_scaling_suspend(struct ufs_hba *hba, bool suspend)
···
 	/* Initialize mutex for exception event control */
 	mutex_init(&hba->ee_ctrl_mutex);
 
+	mutex_init(&hba->wb_mutex);
 	init_rwsem(&hba->clk_scaling_lock);
 
 	ufshcd_init_clk_gating(hba);

include/ufs/ufshcd.h

···
  * @urgent_bkops_lvl: keeps track of urgent bkops level for device
  * @is_urgent_bkops_lvl_checked: keeps track if the urgent bkops level for
  *  device is known or not.
+ * @wb_mutex: used to serialize devfreq and sysfs write booster toggling
  * @clk_scaling_lock: used to serialize device commands and clock scaling
  * @desc_size: descriptor sizes reported by device
  * @scsi_block_reqs_cnt: reference counting for scsi block requests
···
 	enum bkops_status urgent_bkops_lvl;
 	bool is_urgent_bkops_lvl_checked;
 
+	struct mutex wb_mutex;
 	struct rw_semaphore clk_scaling_lock;
 	unsigned char desc_size[QUERY_DESC_IDN_MAX];
 	atomic_t scsi_block_reqs_cnt;