Messages in this thread | | | Subject | Re: [PATCH] scsi: ufs: Fix a possible dead lock in clock scaling | From | Bart Van Assche <> | Date | Wed, 29 Sep 2021 11:15:18 -0700 |
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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.
Thanks,
Bart.
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