Re: [PATCH] [CPUFREQ] fix race condition in store_scaling_governor

[Andrej Gelenberg <andrej.gelenberg@udo.edu>]

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Hi,

i found the cause for the bug: it was commit "[CPUFREQ] remove rwsem 
lock from CPUFREQ_GOV_STOP call (second call site)".
It release the rwsem lock in the middle of __cpufreq_set_policy, that is 
not a good idea. There are no dead lock, which that patch solved,
because cpufreq_ondemand and cpufreq_conservative don't use the rwsem
lock in do_dbs_timer anymore.

Regards,
Andrej Gelenberg

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>From a6f3a23ac011af636e544cd107379bc22c4dba7f Mon Sep 17 00:00:00 2001
From: Andrej Gelenberg <andrej.gelenberg@udo.edu>
Date: Fri, 14 May 2010 12:39:22 +0200
Subject: [PATCH] [CPUFREQ] Revert "[CPUFREQ] remove rwsem lock from CPUFREQ_GOV_STOP call (second call site)"

That patch is not needed, because there is no rwsem lock in cpufreq_ondemand and
cpufreq_conservativ anymore. Lock should not be released until the work done.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=1594

Signed-off-by: Andrej Gelenberg <andrej.gelenberg@udo.edu>
---
 drivers/cpufreq/cpufreq.c |   11 +----------
 1 files changed, 1 insertions(+), 10 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 75d293e..634757f 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1774,17 +1774,8 @@ static int __cpufreq_set_policy(struct cpufreq_policy *data,
 			dprintk("governor switch\n");
 
 			/* end old governor */
-			if (data->governor) {
-				/*
-				 * Need to release the rwsem around governor
-				 * stop due to lock dependency between
-				 * cancel_delayed_work_sync and the read lock
-				 * taken in the delayed work handler.
-				 */
-				unlock_policy_rwsem_write(data->cpu);
+			if (data->governor)
 				__cpufreq_governor(data, CPUFREQ_GOV_STOP);
-				lock_policy_rwsem_write(data->cpu);
-			}
 
 			/* start new governor */
 			data->governor = policy->governor;
-- 
1.7.1