Re: Locking issues with cpufreq and sysfs

[Prarit Bhargava <prarit@redhat.com>]

On 10/13/2014 09:11 AM, Prarit Bhargava wrote:
> There are several issues with the current locking design of cpufreq.  Most
> notably is the panic reported here:
> 
> http://marc.info/?l=linux-kernel&m=140622451625236&w=2
> 
> which was introduced by commit 955ef4833574636819cd269cfbae12f79cbde63a,
> cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT, which introduces
> a race in the changing of the cpufreq policy.  This change was introduced
> because of a lockdep deadlock warning that can be reproduced (on x86 with
> the acpi_cpufreq driver) via the following debug patch:
> 
> iff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
> index b0c18ed..366cfb7 100644
> --- a/drivers/cpufreq/acpi-cpufreq.c
> +++ b/drivers/cpufreq/acpi-cpufreq.c
> @@ -885,6 +885,7 @@ static struct freq_attr *acpi_cpufreq_attr[] = {
> 
>  static struct cpufreq_driver acpi_cpufreq_driver = {
>         .verify         = cpufreq_generic_frequency_table_verify,
> +       .flags          = CPUFREQ_HAVE_GOVERNOR_PER_POLICY,
>         .target_index   = acpi_cpufreq_target,
>         .bios_limit     = acpi_processor_get_bios_limit,
>         .init           = acpi_cpufreq_cpu_init,
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 61190f6..4cb488a 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2195,9 +2195,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *polic
>         /* end old governor */
>         if (old_gov) {
>                 __cpufreq_governor(policy, CPUFREQ_GOV_STOP);
> -               up_write(&policy->rwsem);
>                 __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
> -               down_write(&policy->rwsem);
>         }
> 
>         /* start new governor */
> @@ -2206,9 +2204,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *polic
>                 if (!__cpufreq_governor(policy, CPUFREQ_GOV_START))
>                         goto out;
> 
> -               up_write(&policy->rwsem);
>                 __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
> -               down_write(&policy->rwsem);
>         }
> 
>         /* new governor failed, so re-start old one */
> 
> (which causes the acpi-cpufreq driver to emulate the behaviour of the arm
> cpufreq driver), and by doing
> 
> echo ondemand > /sys/devices/system/cpu/cpu5/cpufreq/scaling_governor
> cat /sys/devices/system/cpu/cpu5/cpufreq/ondemand/*
> echo conservative > /sys/devices/system/cpu/cpu5/cpufreq/scaling_governor
> exit 0
> 
> [Question: was the original reported deadlock "real"?  Did it really happen or
> did lockdep only report it (I may have asked this question previously and
> forgot the answer)?  The reason I ask is that this situation is very similar to
> USB's device removal in which the sysfs attributes are removed for a device but
> not the device it was called for.  I actually think that's part of the problem
> here.]
> 
> The above, obviously, is a complete hack of the code but in a sense does
> mimic a proper locking fix.  However, even with this fix we are still left
> with a race in accessing the sysfs files.  Consider the following example,
> 
> CPU 1: accesses scaling_setspeed to set cpu speed
> 
> simultaneously,
> 
> CPU 2: accesses scaling_governor to set governor to ondemand
> 
> CPU 1 & 2 race ... and this can result in different critical situations.
> The first is that CPU 1 holds the scalling_setspeed open while CPU attempts
> to change the governor.  This results in a syfs warning about creating a
> file with an existing file name which in some cases can lead to additional
> corruption and a panic.  The second case is that CPU 1's setting of the speed
> is now done on the new governor -- which may or may not be correct.  In any
> case an argument could be made that the userspace program doing this type
> of action should be "smart" enough to confirm simultaneous changes... but
> in any case the kernel should not panic or corrupt data.
> 
> The locking is insufficient here, Viresh.  I no longer believe that fixes
> to this locking scheme are the right way to move forward here.  I'm wondering
> if we can look at other alternatives such as maintaining a refcount or
> perhaps using a queuing mechanism for governor and policy related changes.
> 

Uh ... I meant this as "I'm willing to modify the code to do this but I'd like
to know what everyone else thinks before I do anything" ;)

P.

> P.
>