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

[Can Guo]

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.