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

[Adrian Hunter]

On 30/09/2021 06:57, Can Guo wrote:
> On 2021-09-30 02:15, Bart Van Assche wrote:
>> On 9/28/21 8:31 PM, Can Guo wrote:
>>> On 2021-09-18 01:27, Bart Van Assche wrote:
>>>> On 9/16/21 6:51 PM, Can Guo wrote:
>>>>> 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.
>>>>
>>>> Something is wrong with the above description. The downgrade_write() call is
>>>> not followed by down_read() but by up_read(). Additionally, I don't see how
>>>> concurrent calls of ufshcd_devfreq_scale() could lead to a deadlock.
>>>
>>> As mentioned in the commit msg, the down_read() [3] is from ufshcd_wb_ctrl().
>>>
>>> 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
>>>
>>>
>>>> If one thread calls downgrade_write() and another thread calls down_write()
>>>> immediately, that down_write() call will block until the other thread has called up_read()
>>>> without triggering a deadlock.
>>>
>>> Since the down_write() caller is blocked, the down_read() caller, which comes after
>>> down_write(), is blocked too, no? downgrade_write() keeps lock owner as it is, but
>>> it does not change the fact that readers and writers can be blocked by each other.
>>
>> Please use the upstream function names when posting upstream patches.
>> I think that
>> ufshcd_wb_ctrl() has been renamed into ufshcd_wb_toggle().
>>
>> So the deadlock is caused by nested locking - one task holding a
>> reader lock, another
>> task calling down_write() and next the first task grabbing the reader
>> lock recursively?
>> I prefer one of the following two solutions above the patch that has
>> been posted since
>> I expect that both alternatives will result in easier to maintain UFS code:
>> - Fix the down_read() implementation. Making down_read() wait in case
>> of nested locking
>>   seems wrong to me.
>> - Modify the UFS driver such that it does not lock
>> hba->clk_scaling_lock recursively.
> 
> My current change is the 2nd solution - drop the hba->clk_scaling_lock
> before calls ufshcd_wb_toggle() to avoid recursive lock.

I have been looking at elevating hba->clk_scaling_lock to be a general purpose
lock for ufshcd.  That includes allowing down_read if down_write is held already.
The plan being to hold down_write during the error handler instead of releasing it,
which would plug some gaps in synchronization.

So the locking code would look like this:

diff --git a/drivers/scsi/ufs/ufshcd.h b/drivers/scsi/ufs/ufshcd.h
index a42a289eaef93..63b6a26e3a327 100644
--- a/drivers/scsi/ufs/ufshcd.h
+++ b/drivers/scsi/ufs/ufshcd.h
@@ -898,7 +898,8 @@ struct ufs_hba {
 	enum bkops_status urgent_bkops_lvl;
 	bool is_urgent_bkops_lvl_checked;
 
-	struct rw_semaphore clk_scaling_lock;
+	struct rw_semaphore host_rw_sem;
+	struct task_struct *excl_task;
 	unsigned char desc_size[QUERY_DESC_IDN_MAX];
 	atomic_t scsi_block_reqs_cnt;
 
@@ -1418,4 +1419,42 @@ static inline int ufshcd_rpmb_rpm_put(struct ufs_hba *hba)
 	return pm_runtime_put(&hba->sdev_rpmb->sdev_gendev);
 }
 
+static inline void ufshcd_down_read(struct ufs_hba *hba)
+{
+	if (hba->excl_task != current)
+		down_read(&hba->host_rw_sem);
+}
+
+static inline void ufshcd_up_read(struct ufs_hba *hba)
+{
+	if (hba->excl_task != current)
+		up_read(&hba->host_rw_sem);
+}
+
+static inline int ufshcd_down_read_trylock(struct ufs_hba *hba)
+{
+	if (hba->excl_task == current)
+		return 1;
+
+	return down_read_trylock(&hba->host_rw_sem);
+}
+
+static inline void ufshcd_down_write(struct ufs_hba *hba)
+{
+	down_write(&hba->host_rw_sem);
+	hba->excl_task = current;
+}
+
+static inline void ufshcd_up_write(struct ufs_hba *hba)
+{
+	hba->excl_task = NULL;
+	up_write(&hba->host_rw_sem);
+}
+
+static inline void ufshcd_downgrade_write(struct ufs_hba *hba)
+{
+	hba->excl_task = NULL;
+	downgrade_write(&hba->host_rw_sem);
+}
+
 #endif /* End of Header */