[PATCH-next v2 1/2] cpufreq: powernv: Move smp_call_function_any() out of irq safe block

Akshay Adiga akshay.adiga at linux.vnet.ibm.com
Wed May 4 01:19:35 AEST 2016

Fix a WARN_ON caused by smp_call_function_any() when irq is disabled,
because of changes made in the patch ('cpufreq: powernv: Ramp-down
 global pstate slower than local-pstate')

 WARNING: CPU: 0 PID: 4 at kernel/smp.c:291

 Call Trace:
 [c0000007f648f9f0] [c0000007f648fa90] 0xc0000007f648fa90 (unreliable)
 [c0000007f648fa30] [c0000000001430e0] smp_call_function_any+0x170/0x1c0
 [c0000007f648fa90] [c0000000007b4b00]
 [c0000007f648fb00] [c0000000007ac9dc]
 [c0000007f648fbc0] [c0000000007b1b4c] od_dbs_timer+0xcc/0x260
 [c0000007f648fc10] [c0000000007b3024] dbs_work_handler+0x54/0xa0
 [c0000007f648fc50] [c0000000000c49a8] process_one_work+0x1d8/0x590
 [c0000007f648fce0] [c0000000000c4e08] worker_thread+0xa8/0x660
 [c0000007f648fd80] [c0000000000cca88] kthread+0x108/0x130
 [c0000007f648fe30] [c0000000000095e8] ret_from_kernel_thread+0x5c/0x74

- Calling smp_call_function_any() with interrupt disabled (through
 spin_lock_irqsave) could cause a deadlock, as smp_call_function_any()
 relies on the IPI to complete. This is detected in the
 smp_call_function_any() call and hence the WARN_ON.

- As the spinlock (gpstates->lock) is only used to synchronize access of
 global_pstate_info  between timer irq handler and target_index calls. And
 the timer irq handler just try_locks() hence it would not cause a
 deadlock. Hence could do without making spinlocks irq safe.

- As the smp_call_function_any() is a blocking call and does not access
 global_pstates_info, it could reduce the critcal section by moving
 smp_call_function_any() after giving up the lock.

Reported-by: Abdul Haleem <abdhalee at linux.vnet.linux.com>
Signed-off-by: Akshay Adiga <akshay.adiga at linux.vnet.ibm.com>
Patch is based on Rafael's linux-next
 drivers/cpufreq/powernv-cpufreq.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cpufreq.c
index 144c732..1f0e20c 100644
--- a/drivers/cpufreq/powernv-cpufreq.c
+++ b/drivers/cpufreq/powernv-cpufreq.c
@@ -581,9 +581,10 @@ void gpstate_timer_handler(unsigned long data)
 	gpstates->last_gpstate = freq_data.gpstate_id;
 	gpstates->last_lpstate = freq_data.pstate_id;
+	spin_unlock(&gpstates->gpstate_lock);
 	/* Timer may get migrated to a different cpu on cpu hot unplug */
 	smp_call_function_any(policy->cpus, set_pstate, &freq_data, 1);
-	spin_unlock(&gpstates->gpstate_lock);
@@ -596,7 +597,6 @@ static int powernv_cpufreq_target_index(struct cpufreq_policy *policy,
 	struct powernv_smp_call_data freq_data;
 	unsigned int cur_msec, gpstate_id;
-	unsigned long flags;
 	struct global_pstate_info *gpstates = policy->driver_data;
 	if (unlikely(rebooting) && new_index != get_nominal_index())
@@ -607,7 +607,7 @@ static int powernv_cpufreq_target_index(struct cpufreq_policy *policy,
 	cur_msec = jiffies_to_msecs(get_jiffies_64());
-	spin_lock_irqsave(&gpstates->gpstate_lock, flags);
+	spin_lock(&gpstates->gpstate_lock);
 	freq_data.pstate_id = powernv_freqs[new_index].driver_data;
 	if (!gpstates->last_sampled_time) {
@@ -654,13 +654,14 @@ gpstates_done:
 	gpstates->last_gpstate = freq_data.gpstate_id;
 	gpstates->last_lpstate = freq_data.pstate_id;
+	spin_unlock(&gpstates->gpstate_lock);
 	 * Use smp_call_function to send IPI and execute the
 	 * mtspr on target CPU.  We could do that without IPI
 	 * if current CPU is within policy->cpus (core)
 	smp_call_function_any(policy->cpus, set_pstate, &freq_data, 1);
-	spin_unlock_irqrestore(&gpstates->gpstate_lock, flags);
 	return 0;

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