Re: mutex warning in cpufreq + RFC patch

[Stephen Boyd]

On 08/27/13 23:58, Viresh Kumar wrote:
> I haven't gone through the hack yet, but I am trying to understand the
> problem first.. There had been some work in the past around this
> kind of scenarios..
>
> commit 95731ebb114c5f0c028459388560fc2a72fe5049
> Author: Xiaoguang Chen <chenxg@xxxxxxxxxxx>
> Date:   Wed Jun 19 15:00:07 2013 +0800
>
>     cpufreq: Fix governor start/stop race condition
>
>
> The problem probably is poor error checking which is still present at
> few places, in __cpufreq_set_policy() routine..
>
> Can you try after fixing them? Something similar has to be done..
>
> commit 3de9bdeb28638e164d1f0eb38dd68e3f5d2ac95c
> Author: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Date:   Tue Aug 6 22:53:13 2013 +0530
>
>     cpufreq: improve error checking on return values of __cpufreq_governor()

No the problem isn't poor error checking. The problem is between
gov_stop and gov_start userspace can come in and write scaling_min_freq
which will try to acquire the mutex (sorry the copy paste of the error
got messed up so I've repasted it).

WARNING: at kernel/mutex.c:341 __mutex_lock_slowpath+0x14c/0x410()              
DEBUG_LOCKS_WARN_ON(l->magic != l)
Modules linked in:                                                              CPU: 0 PID: 1960 Comm: sh Tainted: G        W    3.10.0 #32                     
[<c010c178>] (unwind_backtrace+0x0/0x11c) from [<c0109dec>] (show_stack+0x10/0x14)
[<c0109dec>] (show_stack+0x10/0x14) from [<c01904cc>] (warn_slowpath_common+0x4c/0x6c)
[<c01904cc>] (warn_slowpath_common+0x4c/0x6c) from [<c019056c>] (warn_slowpath_fmt+0x2c/0x3c)
[<c019056c>] (warn_slowpath_fmt+0x2c/0x3c) from [<c08a0334>] (__mutex_lock_slowpath+0x14c/0x410)
[<c08a0334>] (__mutex_lock_slowpath+0x14c/0x410) from [<c08a0618>] (mutex_lock+0x20/0x3c)
[<c08a0618>] (mutex_lock+0x20/0x3c) from [<c0636114>] (cpufreq_governor_dbs+0x568/0x5f8)
[<c0636114>] (cpufreq_governor_dbs+0x568/0x5f8) from [<c06325b0>] (__cpufreq_governor+0xdc/0x1a4)
[<c06325b0>] (__cpufreq_governor+0xdc/0x1a4) from [<c06328f0>] (__cpufreq_set_policy+0x278/0x2c0)
[<c06328f0>] (__cpufreq_set_policy+0x278/0x2c0) from [<c0632ea0>] (store_scaling_min_freq+0x80/0x9c)
[<c0632ea0>] (store_scaling_min_freq+0x80/0x9c) from [<c0633ae4>] (store+0x58/0x90)
[<c0633ae4>] (store+0x58/0x90) from [<c02a69d4>] (sysfs_write_file+0x100/0x148)
[<c02a69d4>] (sysfs_write_file+0x100/0x148) from [<c0255c18>] (vfs_write+0xcc/0x174)
[<c0255c18>] (vfs_write+0xcc/0x174) from [<c0255f70>] (SyS_write+0x38/0x64)
[<c0255f70>] (SyS_write+0x38/0x64) from [<c0106120>] (ret_fast_syscall+0x0/0x30)


I've applied these patches on top of v3.10

f51e1eb63d9c28cec188337ee656a13be6980cfd (cpufreq: Fix cpufreq regression after suspend/resume
aae760ed21cd690fe8a6db9f3a177ad55d7e12ab (cpufreq: Revert commit a66b2e to fix suspend/resume regression)
e8d05276f236ee6435e78411f62be9714e0b9377 (cpufreq: Revert commit 2f7021a8 to fix CPU hotplug regression) 
2a99859932281ed6c2ecdd988855f8f6838f6743 (cpufreq: Fix cpufreq driver module refcount balance after suspend/resume)
419e172145cf6c51d436a8bf4afcd17511f0ff79 (cpufreq: don't leave stale policy pointer in cdbs->cur_policy)
95731ebb114c5f0c028459388560fc2a72fe5049 (cpufreq: Fix governor start/stop race condition)

That second to last one causes a NULL pointer exception after the mutex
warning above because the limits case does

    if (policy->max < cpu_cdbs->cur_policy->cur)

and that dereferences a NULL cur_policy pointer.

Are there any fixes that I'm missing? I see that some things are
changing in linux-next but they don't look like fixes, more like
optimizations.

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