Message-ID: <CAJZ5v0gHz5cEmX23uXESTmWbx5S+XsXMvxUp4ikTTpgdVy9N3w@mail.gmail.com>
Date:	Tue, 21 Jul 2015 03:14:34 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: Avoid double addition/removal of sysfs links

On Tue, Jul 21, 2015 at 2:47 AM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> Hi Russell,
>
> On Mon, Jul 20, 2015 at 12:36 PM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
>> On Mon, Jul 20, 2015 at 03:17:10PM +0530, Viresh Kumar wrote:
>>> Consider a dual core (0/1) system with two CPUs:
>>> - sharing clock/voltage rails and hence cpufreq-policy
>>> - CPU1 is offline while the cpufreq driver is registered
>>>
>>> - cpufreq_add_dev() is called from subsys callback for CPU0 and we
>>>   create the policy for the CPUs and create link for CPU1.
>>> - cpufreq_add_dev() is called from subsys callback for CPU1, we find
>>>   that the cpu is offline and we try to create a sysfs link for CPU1.
>>> - This results in double addition of the sysfs link and we get this:
>>>
>>>       WARNING: CPU: 0 PID: 1 at fs/sysfs/dir.c:31 sysfs_warn_dup+0x60/0x7c()
>>>       sysfs: cannot create duplicate filename '/devices/system/cpu/cpu1/cpufreq'
>>>       Modules linked in:
>>>       CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.2.0-rc2+ #1704
>>>       Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
>>>       Backtrace:
>>>       [<c0013248>] (dump_backtrace) from [<c00133e4>] (show_stack+0x18/0x1c)
>>>        r6:c01a1f30 r5:0000001f r4:00000000 r3:00000000
>>>       [<c00133cc>] (show_stack) from [<c076920c>] (dump_stack+0x7c/0x98)
>>>       [<c0769190>] (dump_stack) from [<c0029ab4>] (warn_slowpath_common+0x80/0xbc)
>>>        r4:d74abbd0 r3:d74c0000
>>>       [<c0029a34>] (warn_slowpath_common) from [<c0029b94>] (warn_slowpath_fmt+0x38/0x40)
>>>        r8:ffffffef r7:00000000 r6:d75a8960 r5:c0993280 r4:d6b4d000
>>>       [<c0029b60>] (warn_slowpath_fmt) from [<c01a1f30>] (sysfs_warn_dup+0x60/0x7c)
>>>        r3:d6b4dfe7 r2:c0930750
>>>       [<c01a1ed0>] (sysfs_warn_dup) from [<c01a22c8>] (sysfs_do_create_link_sd+0xb8/0xc0)
>>>        r6:d75a8960 r5:c0993280 r4:d00aba20
>>>       [<c01a2210>] (sysfs_do_create_link_sd) from [<c01a22fc>] (sysfs_create_link+0x2c/0x3c)
>>>        r10:00000001 r8:c14db3c8 r7:d7b89010 r6:c0ae7c60 r5:d7b89010 r4:d00d1200
>>>       [<c01a22d0>] (sysfs_create_link) from [<c0506160>] (add_cpu_dev_symlink+0x34/0x5c)
>>>       [<c050612c>] (add_cpu_dev_symlink) from [<c05084d0>] (cpufreq_add_dev+0x674/0x794)
>>>        r5:00000001 r4:00000000
>>>       [<c0507e5c>] (cpufreq_add_dev) from [<c03db114>] (subsys_interface_register+0x8c/0xd0)
>>>        r10:00000003 r9:d7bb01f0 r8:c14db3c8 r7:00106738 r6:c0ae7c60 r5:c0acbd08
>>>        r4:c0ae7e20
>>>       [<c03db088>] (subsys_interface_register) from [<c0508a2c>] (cpufreq_register_driver+0x104/0x1f4)
>>>
>>>
>>> The check for offline-cpu in cpufreq_add_dev() is present to ensure that
>>> link gets added for the CPUs, that weren't physically present earlier
>>> and we missed the case where a CPU is offline while registering the
>>> driver.
>>>
>>> Fix this by keeping track of CPUs for which link is already created, and
>>> avoiding duplicate sysfs entries.
>>
>> Why do we try to create the symlink for CPU devices which we haven't
>> "detected" yet (iow, we haven't had cpufreq_add_dev() called for)?
>> Surely we are guaranteed to have cpufreq_add_dev() called for every
>> CPU which exists in sysfs?  So why not _only_ create the sysfs symlinks
>> when cpufreq_add_dev() is notified that a CPU subsys interface is
>> present?
>
> That's something I've overlooked.
>
> Yes, we should be doing exactly that.
>
>> Sure, if the policy changes, we need to do maintanence on these symlinks,
>> but I see only one path down into cpufreq_add_dev_symlink(), which is:
>>
>>         cpufreq_add_dev() -> cpufreq_add_dev_interface() ->
>>                 cpufreq_add_dev_symlink()
>>
>> In other words, only when we see a new CPU interface appears, not when
>> the policy changes.  If the set of related CPUs is policy independent,
>> why is this information carried in the cpufreq_policy struct?
>
> It is not policy-dependent, but the way that information is gathered
> is not exactly straightforward.  It generally depends on what the
> platform firmware tells us about the topology.
>
>> If it is policy dependent, then I see no code which handles the effect
>> of a policy change where the policy->related_cpus is different.  To me,
>> that sounds like a rather huge design hole.
>>
>> Things get worse.  Reading drivers/base/cpu.c, CPU interface nodes are
>> only ever created - they're created for the set of _possible_ CPUs in
>> the system, not those which are possible and present, and there is no
>> unregister_cpu() API, only a register_cpu() API.
>
> There is unregister_cpu() API too, but it is called from
> arch_unregister_cpu().  And it calls device_unregister() and all of
> the appropriate things happen AFAICS.  Eventually,
> cpufreq_remove_dev() is called from that path.

That said, cpu_present_mask may only be updated after calling
arch_unregister_cpu(), so checking it in cpufreq_remove_dev() doesn't
really help.

It looks like using cpufreq_remove_dev() as the subsys ->remove_dev
callback is a mistake as it cannot really tell the difference between
that code path and the CPU offline one.

Thanks,
Rafael
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