From b14893a62c73af0eca414cfed505b8c09efc613c Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Sun, 17 May 2009 10:30:45 -0400 Subject: [CPUFREQ] fix timer teardown in ondemand governor * Rafael J. Wysocki (rjw@sisk.pl) wrote: > This message has been generated automatically as a part of a report > of regressions introduced between 2.6.28 and 2.6.29. > > The following bug entry is on the current list of known regressions > introduced between 2.6.28 and 2.6.29. Please verify if it still should > be listed and let me know (either way). > > > Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=13186 > Subject : cpufreq timer teardown problem > Submitter : Mathieu Desnoyers > Date : 2009-04-23 14:00 (24 days old) > References : http://marc.info/?l=linux-kernel&m=124049523515036&w=4 > Handled-By : Mathieu Desnoyers > Patch : http://patchwork.kernel.org/patch/19754/ > http://patchwork.kernel.org/patch/19753/ > (updated changelog) cpufreq fix timer teardown in ondemand governor The problem is that dbs_timer_exit() uses cancel_delayed_work() when it should use cancel_delayed_work_sync(). cancel_delayed_work() does not wait for the workqueue handler to exit. The ondemand governor does not seem to be affected because the "if (!dbs_info->enable)" check at the beginning of the workqueue handler returns immediately without rescheduling the work. The conservative governor in 2.6.30-rc has the same check as the ondemand governor, which makes things usually run smoothly. However, if the governor is quickly stopped and then started, this could lead to the following race : dbs_enable could be reenabled and multiple do_dbs_timer handlers would run. This is why a synchronized teardown is required. The following patch applies to, at least, 2.6.28.x, 2.6.29.1, 2.6.30-rc2. Depends on patch cpufreq: remove rwsem lock from CPUFREQ_GOV_STOP call Signed-off-by: Mathieu Desnoyers CC: Andrew Morton CC: gregkh@suse.de CC: stable@kernel.org CC: cpufreq@vger.kernel.org CC: Ingo Molnar CC: rjw@sisk.pl CC: Ben Slusky Signed-off-by: Dave Jones --- drivers/cpufreq/cpufreq_ondemand.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'drivers') diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c index 338f428..e741c33 100644 --- a/drivers/cpufreq/cpufreq_ondemand.c +++ b/drivers/cpufreq/cpufreq_ondemand.c @@ -98,6 +98,9 @@ static unsigned int dbs_enable; /* number of CPUs using this policy */ * (like __cpufreq_driver_target()) is being called with dbs_mutex taken, then * cpu_hotplug lock should be taken before that. Note that cpu_hotplug lock * is recursive for the same process. -Venki + * DEADLOCK ALERT! (2) : do_dbs_timer() must not take the dbs_mutex, because it + * would deadlock with cancel_delayed_work_sync(), which is needed for proper + * raceless workqueue teardown. */ static DEFINE_MUTEX(dbs_mutex); @@ -562,7 +565,7 @@ static inline void dbs_timer_init(struct cpu_dbs_info_s *dbs_info) static inline void dbs_timer_exit(struct cpu_dbs_info_s *dbs_info) { dbs_info->enable = 0; - cancel_delayed_work(&dbs_info->work); + cancel_delayed_work_sync(&dbs_info->work); } static int cpufreq_governor_dbs(struct cpufreq_policy *policy, -- cgit v1.1