On Wednesday, December 24, 2014 02:37:14 PM Nishanth Menon wrote: > On 12/24/2014 11:44 AM, Dmitry Torokhov wrote: > > On Wed, Dec 24, 2014 at 9:37 AM, Nishanth Menon <n...@ti.com> wrote: > >> On 12/24/2014 11:31 AM, Dmitry Torokhov wrote: > >>> On Wed, Dec 24, 2014 at 9:16 AM, Nishanth Menon <n...@ti.com> wrote: > >>>> On 12/24/2014 11:09 AM, Dmitry Torokhov wrote: > >>>>> On Wed, Dec 24, 2014 at 8:48 AM, Nishanth Menon <n...@ti.com> wrote: > >>>>>> On 12/16/2014 05:09 PM, Dmitry Torokhov wrote: > >>>>>>> A lot of callers are missing the fact that dev_pm_opp_get_opp_count > >>>>>>> needs to be called under RCU lock. Given that RCU locks can safely be > >>>>>>> nested, instead of providing *_locked() API, let's take RCU lock > >>>>>>> inside > >>>>>>> dev_pm_opp_get_opp_count() and leave callers as is. > >>>>>> > >>>>>> While it is true that we can safely do nested RCU locks, This also > >>>>>> encourages wrong usage. > >>>>>> > >>>>>> count = dev_pm_opp_get_opp_count(dev) > >>>>>> ^^ point A > >>>>>> array = kzalloc(count * sizeof (*array)); > >>>>>> rcu_read_lock(); > >>>>>> ^^ point B > >>>>>> .. work down the list and add OPPs.. > >>>>>> ... > >>>>>> > >>>>>> Between A and B, we might have had list modification (dynamic OPP > >>>>>> addition or deletion) - which implies that the count is no longer > >>>>>> accurate between point A and B. instead, enforcing callers to have the > >>>>>> responsibility of rcu_lock is exactly what we have to do since the OPP > >>>>>> library has no clue how to enforce pointer or data accuracy. > >>>>> > >>>>> No, you seem to have a misconception that rcu_lock protects you past > >>>>> the point B, but that is also wrong. The only thing rcu "lock" > >>>>> provides is safe traversing the list and guarantee that elements will > >>>>> not disappear while you are referencing them, but list can both > >>>>> contract and expand under you. In that regard code in > >>>>> drivers/cpufreq/cpufreq_opp.c is utterly wrong. If you want to count > >>>>> the list and use number of elements you should be taking a mutex. > >>>>> Luckily all cpufreq drivers at the moment only want to see if OPP > >>>>> table is empty or not, so as a stop-gap we can take rcu_lock > >>>>> automatically as we are getting count. We won't get necessarily > >>>>> accurate result, but at least we will be safe traversing the list. > >>>> > >>>> So, instead of a half solution, lets consider this in the realm of > >>>> dynamic OPPs as well. agreed to the point that we only have safe > >>>> traversal and pointer validity. the real problem however is with > >>>> "dynamic OPPs" (one of the original reasons why i did not add dynamic > >>>> OPPs in the original version was to escape from it's complexity for > >>>> users - anyways.. we are beyond that now). if OPPs can be removed on > >>>> the fly, we need the following: > >>>> a) use OPP notifiers to adequately handle list modification > >>>> b) lock down list modification (and associated APIs) to ensure that > >>>> the original cpufreq /devfreq list is correct. > >>>> > >>>> I still dont see the need to do this half solution. > >>> > >>> The need for half solution at the moment is that you can't safely > >>> travel the lists and may crash on an invalid pointer. > >> > >> So, fix the cpufreq-dt instead of moving the hack inside OPP driver. > > > > I started there, but it is not only cpufreq-dt that got it wrong. I > > considered changing individual drivers (Viresh also suggested adding > > _locked() variant API), but decided patching opp was less invasive for > > now. > > True. I had done an audit and cleanup, I think a couple or so years > back and things ofcourse tend to go down the bitrot path without > constant checkups :( > > >>> Going forward I think (I mentioned that in my other email) that we > >>> should rework the OPP API so that callers fetch OPP table object for a > >>> device at init/probe time and then use it to get OPPs. This way won't > >>> have to travel two lists any time we want to reference an OPP. > >>> > >>> And instead of relying notifiers, maybe look into using OPP tables > >>> directly in cpufreq drivers instead of converting OPP into static-ish > >>> cpufreq tables. > >>> > >> > >> If you'd like a proper fix for OPP usage, I am all open to see such a > >> proposal that works not just for cpufreq, but also for devfreq as well. > > > > Yeah, let's see what kind of time I have ;) > > That would be nice. Thank you. > > if you could post the split off the remaining patches from the series > esp patches #1 and #2 w.r.t 3.19-rc1 and repost, it will be nice to > get them merged in as they do look like obvious improvements good to > get in without depending on the remainder of the series which we can > work on meanwhile.
I actually have the entire Dmitry's series queued up for the next push (most likely on Monday) and at this point I'm not seeing a compelling reason for not pushing it. -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
