lkml.org 
[lkml]   [2009]   [Jul]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRE: [patch 1/4] cpufreq: Eliminate the recent lockdep warnings in cpufreq


    >-----Original Message-----
    >From: Mathieu Desnoyers [mailto:mathieu.desnoyers@polymtl.ca]
    >Sent: Thursday, July 02, 2009 6:07 PM
    >To: Pallipadi, Venkatesh
    >Cc: Dave Jones; linux-kernel@vger.kernel.org;
    >cpufreq@vger.kernel.org; kernel-testers@vger.kernel.org; Ingo
    >Molnar; Rafael J. Wysocki; Dave Young; Pekka Enberg; Thomas Renninger
    >Subject: Re: [patch 1/4] cpufreq: Eliminate the recent lockdep
    >warnings in cpufreq
    >
    >* venkatesh.pallipadi@intel.com (venkatesh.pallipadi@intel.com) wrote:
    >> Commit b14893a62c73af0eca414cfed505b8c09efc613c although it was very
    >> much needed to properly cleanup ondemand timer, opened-up a
    >can of worms
    >> related to locking dependencies in cpufreq.
    >>
    >> Patch here defines the need for dbs_mutex and cleans up its usage in
    >> ondemand governor. This also resolves the lockdep warnings
    >reported here
    >>
    >> http://lkml.indiana.edu/hypermail/linux/kernel/0906.1/01925.html
    >> http://lkml.indiana.edu/hypermail/linux/kernel/0907.0/00820.html
    >>
    >> and few others..
    >>
    >> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
    >> ---
    >> drivers/cpufreq/cpufreq.c | 4 ++--
    >> drivers/cpufreq/cpufreq_conservative.c | 27
    >+++++++++++----------------
    >> drivers/cpufreq/cpufreq_ondemand.c | 27
    >+++++++++++----------------
    >> 3 files changed, 24 insertions(+), 34 deletions(-)
    >>
    >> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
    >> index 6e2ec0b..c7fe16e 100644
    >> --- a/drivers/cpufreq/cpufreq.c
    >> +++ b/drivers/cpufreq/cpufreq.c
    >> @@ -1070,8 +1070,6 @@ static int __cpufreq_remove_dev(struct
    >sys_device *sys_dev)
    >> spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
    >> #endif
    >>
    >> - unlock_policy_rwsem_write(cpu);
    >> -
    >> if (cpufreq_driver->target)
    >> __cpufreq_governor(data, CPUFREQ_GOV_STOP);
    >>
    >> @@ -1088,6 +1086,8 @@ static int __cpufreq_remove_dev(struct
    >sys_device *sys_dev)
    >> if (cpufreq_driver->exit)
    >> cpufreq_driver->exit(data);
    >>
    >> + unlock_policy_rwsem_write(cpu);
    >> +
    >> free_cpumask_var(data->related_cpus);
    >> free_cpumask_var(data->cpus);
    >> kfree(data);
    >> diff --git a/drivers/cpufreq/cpufreq_conservative.c
    >b/drivers/cpufreq/cpufreq_conservative.c
    >> index 7fc58af..58889f2 100644
    >> --- a/drivers/cpufreq/cpufreq_conservative.c
    >> +++ b/drivers/cpufreq/cpufreq_conservative.c
    >> @@ -70,15 +70,10 @@ static DEFINE_PER_CPU(struct
    >cpu_dbs_info_s, cpu_dbs_info);
    >> static unsigned int dbs_enable; /* number of CPUs using
    >this policy */
    >>
    >> /*
    >> - * DEADLOCK ALERT! There is a ordering requirement between
    >cpu_hotplug
    >> - * lock and dbs_mutex. cpu_hotplug lock should always be held before
    >> - * dbs_mutex. If any function that can potentially take
    >cpu_hotplug lock
    >> - * (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.
    >> + * dbs_mutex protects data in dbs_tuners_ins from
    >concurrent changes on
    >> + * different CPUs. It protects dbs_enable in governor
    >start/stop. It also
    >> + * serializes governor limit_change with do_dbs_timer. We
    >do not want
    >> + * do_dbs_timer to run when user is changing the governor or limits.
    >> */
    >> static DEFINE_MUTEX(dbs_mutex);
    >>
    >> @@ -488,18 +483,17 @@ static void do_dbs_timer(struct
    >work_struct *work)
    >>
    >> delay -= jiffies % delay;
    >>
    >> - if (lock_policy_rwsem_write(cpu) < 0)
    >> - return;
    >> + mutex_lock(&dbs_mutex);
    >
    >OK, I now have absolutely no idea what the rwsem mutex is protecting
    >anymore.
    >
    >You should probably describe the new world order not just in terms of
    >what the dbs_mutex is protecting, but also about what the rwsem is
    >doing. I'm worried that this rwsem is there to protect against
    >more than
    >what is protected by the dbs_mutex local to the ondemand/conservative
    >governors.
    >
    >See below,
    >
    >>
    >> if (!dbs_info->enable) {
    >> - unlock_policy_rwsem_write(cpu);
    >> + mutex_unlock(&dbs_mutex);
    >> return;
    >> }
    >>
    >> dbs_check_cpu(dbs_info);
    >>
    >> queue_delayed_work_on(cpu, kconservative_wq,
    >&dbs_info->work, delay);
    >> - unlock_policy_rwsem_write(cpu);
    >> + mutex_unlock(&dbs_mutex);
    >> }
    >>
    >> static inline void dbs_timer_init(struct cpu_dbs_info_s *dbs_info)
    >> @@ -590,15 +584,16 @@ static int cpufreq_governor_dbs(struct
    >cpufreq_policy *policy,
    >> &dbs_cpufreq_notifier_block,
    >> CPUFREQ_TRANSITION_NOTIFIER);
    >> }
    >> - dbs_timer_init(this_dbs_info);
    >> -
    >> mutex_unlock(&dbs_mutex);
    >>
    >> + dbs_timer_init(this_dbs_info);
    >> +
    >> break;
    >>
    >> case CPUFREQ_GOV_STOP:
    >> - mutex_lock(&dbs_mutex);
    >> dbs_timer_exit(this_dbs_info);
    >
    >So now the only thing that seems to prevent the init and exit to race
    >with each other is the rwsem. But this does not seem to be described
    >anywhere.

    Mathieu,

    Yes. rwsem in cpufreq core makes sure that START and STOP happen sequentially. There
    Is no way for START and STOP for a CPU to happen at the same time as cpufreq core holds
    per policy rwsem lock before making any change to the policy. I can add a comment to
    that effect in cpufreq.c. This is a clean seperation across cpufreq core and governor,
    as cpufreq core takes care of all the policy changes. With that, do you see any
    Issues/races with this patchset?

    Thanks,
    Venki

    \
     
     \ /
      Last update: 2009-07-03 04:05    [W:0.049 / U:1.204 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site