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

[Can Guo <cang@codeaurora.org>]

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.

Thanks,

Can Guo.

> 
> Thanks,
> 
> Bart.