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

[Bart Van Assche]

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. 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.

Thanks,

Bart.