[PATCH] cpufreq, release policy->rwsem on error

[Prarit Bhargava <prarit@redhat.com>]

I have not been able to reliably reproduce the hardware failure that caused
the warning.  In order to reproduce this I resorted to (sorry for the
cut-and-paste)

	diff --git a/drivers/cpufreq/pcc-cpufreq.c b/drivers/cpufreq/pcc-cpufreq.c
	index 728a2d8..c7724d2 100644
	--- a/drivers/cpufreq/pcc-cpufreq.c
	+++ b/drivers/cpufreq/pcc-cpufreq.c
	@@ -146,6 +146,8 @@ static unsigned int pcc_get_freq(unsigned int cpu)
		u32 input_buffer;
		u32 output_buffer;

	+       return 0;
	+
		spin_lock(&pcc_lock);

		pr_debug("get: get_freq for CPU %d\n", cpu);

to simulate the cpufreq_driver->get() hardware failure.

---8<---

While debugging a cpufreq-related hardware failure on a system I saw the
following lockdep warning:

 =========================
 [ BUG: held lock freed! ] 3.17.0-rc4+ #1 Tainted: G            E
 -------------------------
 insmod/2247 is freeing memory ffff88006e1b1400-ffff88006e1b17ff, with a lock still held there!
  (&policy->rwsem){+.+...}, at: [<ffffffff8156d37d>] __cpufreq_add_dev.isra.21+0x47d/0xb80
 3 locks held by insmod/2247:
  #0:  (subsys mutex#5){+.+.+.}, at: [<ffffffff81485579>] subsys_interface_register+0x69/0x120
  #1:  (cpufreq_rwsem){.+.+.+}, at: [<ffffffff8156cf73>] __cpufreq_add_dev.isra.21+0x73/0xb80
  #2:  (&policy->rwsem){+.+...}, at: [<ffffffff8156d37d>] __cpufreq_add_dev.isra.21+0x47d/0xb80

 stack backtrace:
 CPU: 0 PID: 2247 Comm: insmod Tainted: G            E  3.17.0-rc4+ #1
 Hardware name: HP ProLiant MicroServer Gen8, BIOS J06 08/24/2013
  0000000000000000 000000008f3063c4 ffff88006f87bb30 ffffffff8171b358
  ffff88006bcf3750 ffff88006f87bb68 ffffffff810e09e1 ffff88006e1b1400
  ffffea0001b86c00 ffffffff8156d327 ffff880073003500 0000000000000246
 Call Trace:
  [<ffffffff8171b358>] dump_stack+0x4d/0x66
  [<ffffffff810e09e1>] debug_check_no_locks_freed+0x171/0x180
  [<ffffffff8156d327>] ? __cpufreq_add_dev.isra.21+0x427/0xb80
  [<ffffffff8121412b>] kfree+0xab/0x2b0
  [<ffffffff8156d327>] __cpufreq_add_dev.isra.21+0x427/0xb80
  [<ffffffff81724cf7>] ? _raw_spin_unlock+0x27/0x40
  [<ffffffffa003517f>] ? pcc_cpufreq_do_osc+0x17f/0x17f [pcc_cpufreq]
  [<ffffffff8156da8e>] cpufreq_add_dev+0xe/0x10
  [<ffffffff814855d1>] subsys_interface_register+0xc1/0x120
  [<ffffffff8156bcf2>] cpufreq_register_driver+0x112/0x340
  [<ffffffff8121415a>] ? kfree+0xda/0x2b0
  [<ffffffffa003517f>] ? pcc_cpufreq_do_osc+0x17f/0x17f [pcc_cpufreq]
  [<ffffffffa003562e>] pcc_cpufreq_init+0x4af/0xe81 [pcc_cpufreq]
  [<ffffffffa003517f>] ? pcc_cpufreq_do_osc+0x17f/0x17f [pcc_cpufreq]
  [<ffffffff81002144>] do_one_initcall+0xd4/0x210
  [<ffffffff811f7472>] ? __vunmap+0xd2/0x120
  [<ffffffff81127155>] load_module+0x1315/0x1b70
  [<ffffffff811222a0>] ? store_uevent+0x70/0x70
  [<ffffffff811229d9>] ? copy_module_from_fd.isra.44+0x129/0x180
  [<ffffffff81127b86>] SyS_finit_module+0xa6/0xd0
  [<ffffffff81725b69>] system_call_fastpath+0x16/0x1b
 cpufreq: __cpufreq_add_dev: ->get() failed
insmod: ERROR: could not insert module pcc-cpufreq.ko: No such device

The warning occurs in the __cpufreq_add_dev() code which does

        down_write(&policy->rwsem);
	...
        if (cpufreq_driver->get && !cpufreq_driver->setpolicy) {
                policy->cur = cpufreq_driver->get(policy->cpu);
                if (!policy->cur) {
                        pr_err("%s: ->get() failed\n", __func__);
                        goto err_get_freq;
                }

If cpufreq_driver->get(policy->cpu) returns an error we execute the
code at err_get_freq, which does not up the policy->rwsem.  This causes
the lockdep warning.

Trivial patch to up the policy->rwsem in the error path.

After the patch has been applied, and an error occurs in the
cpufreq_driver->get(policy->cpu) call we will now see

cpufreq: __cpufreq_add_dev: ->get() failed
cpufreq: __cpufreq_add_dev: ->get() failed
modprobe: ERROR: could not insert 'pcc_cpufreq': No such device

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linux-pm@vger.kernel.org
---
 drivers/cpufreq/cpufreq.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index d9fdedd..aadaed0 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1289,6 +1289,8 @@ err_get_freq:
 		per_cpu(cpufreq_cpu_data, j) = NULL;
 	write_unlock_irqrestore(&cpufreq_driver_lock, flags);
 
+	up_write(&policy->rwsem);
+
 	if (cpufreq_driver->exit)
 		cpufreq_driver->exit(policy);
 err_set_policy_cpu:
-- 
1.7.9.3