From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
Linux PM list <linux-pm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/12] cpufreq: governor: Close dbs_data update race condition
Date: Thu, 18 Feb 2016 17:20:52 +0100 [thread overview]
Message-ID: <CAJZ5v0jFP5hdcomAw1GcMgVAhw1mnwpZ3vdM7vN59SFkiug_YA@mail.gmail.com> (raw)
In-Reply-To: <20160218052409.GG2610@vireshk-i7>
On Thu, Feb 18, 2016 at 6:24 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> On 18-02-16, 02:19, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>
>> It is possible for a dbs_data object to be updated after its
>> usage counter has become 0. That may happen if governor_store()
>> runs (via a govenor tunable sysfs attribute write) in parallel
>> with cpufreq_governor_exit() called for the last cpufreq policy
>> associated with the dbs_data in question. In that case, if
>> governor_store() acquires dbs_data->mutex right after
>> cpufreq_governor_exit() has released it, the ->store() callback
>> invoked by it may operate on dbs_data with no users. Although
>> sysfs will cause the kobject_put() in cpufreq_governor_exit() to
>> block until governor_store() has returned, that situation may
>> lead to some unexpected results, depending on the implementation
>> of the ->store callback, and therefore it should be avoided.
>>
>> To that end, modify governor_store() to check the dbs_data's
>> usage count before invoking the ->store() callback and return
>> an error if it is 0 at that point.
>>
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> ---
>> drivers/cpufreq/cpufreq_governor.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> Index: linux-pm/drivers/cpufreq/cpufreq_governor.c
>> ===================================================================
>> --- linux-pm.orig/drivers/cpufreq/cpufreq_governor.c
>> +++ linux-pm/drivers/cpufreq/cpufreq_governor.c
>> @@ -112,7 +112,7 @@ static ssize_t governor_store(struct kob
>>
>> mutex_lock(&dbs_data->mutex);
>>
>> - if (gattr->store)
>> + if (dbs_data->usage_count && gattr->store)
>
> That's not gonna be enough. The above lock doesn't guarantee
> protection with any such races.
I'm not really sure what you're talking about to be honest, so please
be more specific. You can say "For example, function X decrements the
usage count without locking" or similar.
Such vague comments are quite difficult to address, especially if they
don't hold any water. :-)
> And so usage_count can become zero
> just after this check.
But how?
The only place it is decremented is cpufreq_governor_exit() and there
it is done under dbs_data->mutex (at my direct request, BTW). So we
are guaranteed that it won't go down to zero while we're holding
dbs_data->mutex, aren't we?
> Btw, we should also kill the gattr->store checks here as well, as we
> did it in cpufreq-core.
>
>> ret = gattr->store(dbs_data, buf, count);
>>
>> mutex_unlock(&dbs_data->mutex);
Yeah, they are quite useless. But not in this patch.
Thanks,
Rafael
next prev parent reply other threads:[~2016-02-18 16:20 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-18 1:17 [PATCH 0/12] cpufreq: More governor code reorganization Rafael J. Wysocki
2016-02-18 1:19 ` [PATCH 1/12] cpufreq: governor: Close dbs_data update race condition Rafael J. Wysocki
2016-02-18 5:24 ` Viresh Kumar
2016-02-18 16:20 ` Rafael J. Wysocki [this message]
2016-02-19 2:27 ` Viresh Kumar
2016-02-19 2:34 ` Rafael J. Wysocki
2016-02-19 3:09 ` Viresh Kumar
2016-02-18 1:20 ` [PATCH 2/12] cpufreq: governor: Move io_is_busy to struct dbs_data Rafael J. Wysocki
2016-02-18 5:28 ` Viresh Kumar
2016-02-18 1:21 ` [PATCH 3/12] cpufreq: governor: Add a ->start callback for governors Rafael J. Wysocki
2016-02-18 5:36 ` Viresh Kumar
2016-02-18 1:22 ` [PATCH 4/12] cpufreq: governor: Drop unused governor callback and data fields Rafael J. Wysocki
2016-02-18 5:37 ` Viresh Kumar
2016-02-18 1:24 ` [PATCH 5/12] cpufreq: ondemand: Drop one more callback from struct od_ops Rafael J. Wysocki
2016-02-18 5:38 ` Viresh Kumar
2016-02-18 1:26 ` [PATCH 6/12] cpufreq: governor: Fix CPU load information updates via ->store Rafael J. Wysocki
2016-02-18 5:44 ` Viresh Kumar
2016-02-18 17:37 ` Rafael J. Wysocki
2016-02-18 1:28 ` [PATCH 7/12] cpufreq: ondemand: Rework the handling of powersave bias updates Rafael J. Wysocki
2016-02-18 5:53 ` Viresh Kumar
2016-02-18 1:30 ` [PATCH 8/12] cpufreq: governor: Make governor private data per-policy Rafael J. Wysocki
2016-02-18 6:03 ` Viresh Kumar
2016-02-18 17:56 ` [PATCH v2 " Rafael J. Wysocki
2016-02-19 2:36 ` Viresh Kumar
2016-02-18 1:31 ` [PATCH 9/12] cpufreq: governor: Move per-CPU data to the common code Rafael J. Wysocki
2016-02-18 6:08 ` Viresh Kumar
2016-02-18 1:32 ` [PATCH 10/12] cpufreq: governor: Relocate definitions of tuners structures Rafael J. Wysocki
2016-02-18 6:09 ` Viresh Kumar
2016-02-18 17:57 ` [PATCH v2 " Rafael J. Wysocki
2016-02-19 2:36 ` Viresh Kumar
2016-02-18 1:33 ` [PATCH 11/12] cpufreq: governor: Make dbs_data_mutex static Rafael J. Wysocki
2016-02-18 6:09 ` Viresh Kumar
2016-02-18 1:38 ` [PATCH 12/12] cpufreq: governor: Narrow down the dbs_data_mutex coverage Rafael J. Wysocki
2016-02-18 6:20 ` Viresh Kumar
2016-02-18 16:32 ` Rafael J. Wysocki
2016-02-18 17:58 ` [PATCH v2 " Rafael J. Wysocki
2016-02-19 2:38 ` Viresh Kumar
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