Re: [PATCH 0/3] cpufreq: governor: Fix potential races

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 06/03/2015 03:57 PM, Viresh Kumar wrote:
> Hi Rafael,
> 
> Preeti recently highlighted [1] some issues in cpufreq core locking with
> respect to governors. I wanted to solve them after we have simplified
> the hotplug paths in cpufreq core with my latest patches, but now that
> she has poked me, I have done some work in that area.
> 
> I am trying to solve only a part of the bigger problem (in a way that I
> feel is the right way ahead). The first patches restructures code to
> make it more readable and the last patch does all the major changes. The
> logs in that one should be good enough to explain why and what I am
> doing.
> 
> The first two shouldn't bring any functional change and so can be
> applied early if you are confident about them.
> 
> @Preeti: I would like you to test these patches. These should get rid of
> the crashes you were facing but may generate a WARN() from line 447 of
> cpufreq_governor.c, if the sequence is wrong. That has to be fixed
> separately.
> 
> Line 447: WARN_ON(!dbs_data && (event != CPUFREQ_GOV_POLICY_INIT))
> 
> Rebased over: v4.1-rc6
> Tested-on: ARM dual Cortex -A15 Exynos board.
> 
> [1] http://marc.info/?i=20150601064031.2972.59208.stgit%40perfhull-ltc.austin.ibm.com
> 
> Viresh Kumar (3):
>   cpufreq: governor: register notifier from cs_init()
>   cpufreq: governor: split cpufreq_governor_dbs()
>   cpufreq: governor: Serialize governor callbacks
> 
>  drivers/cpufreq/cpufreq_conservative.c |  28 +--
>  drivers/cpufreq/cpufreq_governor.c     | 340 ++++++++++++++++++---------------
>  drivers/cpufreq/cpufreq_governor.h     |  16 +-
>  drivers/cpufreq/cpufreq_ondemand.c     |   6 +-
>  4 files changed, 209 insertions(+), 181 deletions(-)
> 

I did a hotplug test on a single core alongside changing governors
between ondemand and conservative on the same core. The policy is per
core on powerpc. Within a second of that run the kernel panics. The
backtrace is below:

[  165.981836] Unable to handle kernel paging request for data at
address 0x00000000
[  165.981929] Faulting instruction address: 0xc00000000053b3e0
cpu 0x4: Vector: 300 (Data Access) at [c000000fe0b2b880]
    pc: c00000000053b3e0: __bitmap_weight+0x70/0x100
    lr: c00000000085a008: need_load_eval+0x38/0xf0
    sp: c000000fe0b2bb00
   msr: 9000000100009033
   dar: 0
 dsisr: 40000000
  current = 0xc000000003e4fc90
  paca    = 0xc000000007da2600	 softe: 0	 irq_happened: 0x01
    pid   = 812, comm = kworker/4:2
enter ? for help
[c000000fe0b2bb50] c00000000085a008 need_load_eval+0x38/0xf0
[c000000fe0b2bb80] c00000000085815c cs_dbs_timer+0xdc/0x150
[c000000fe0b2bbe0] c0000000000f489c process_one_work+0x24c/0x910
[c000000fe0b2bc90] c0000000000f50dc worker_thread+0x17c/0x540
[c000000fe0b2bd20] c0000000000fed70 kthread+0x120/0x140
[c000000fe0b2be30] c000000000009678 ret_from_kernel_thread+0x5c/0x64

The crash is the same as was reported at
http://www.gossamer-threads.com/lists/linux/kernel/2186336.

Regards
Preeti U Murthy