Locking issues with cpufreq and sysfs

[Prarit Bhargava]

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.

P.
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