Messages in this thread | | | Date | Thu, 30 Sep 2021 11:57:38 +0800 | From | Can Guo <> | Subject | Re: [PATCH] scsi: ufs: Fix a possible dead lock in clock scaling |
| |
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.
| |