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

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Date:   Wed, 29 Sep 2021 11:31:16 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     asutoshd@...eaurora.org, nguyenb@...eaurora.org,
        hongwus@...eaurora.org, ziqichen@...eaurora.org,
        linux-scsi@...r.kernel.org, kernel-team@...roid.com,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Bean Huo <beanhuo@...ron.com>,
        Stanley Chu <stanley.chu@...iatek.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scsi: ufs: Fix a possible dead lock in clock scaling

Hi Bart,

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.

> 
> Thanks,
> 
> Bart.

Thanks,

Can.