Re: [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Friday, February 05, 2016 03:19:25 PM Viresh Kumar wrote:
> On 05-02-16, 04:54, Rafael J. Wysocki wrote:
> > Having actually posted that series again after cleaning it up I can say
> > what I'm thinking about hopefully without confusing anyone too much.  So
> > please bear in mind that I'm going to refer to this series below:
> > 
> > http://marc.info/?l=linux-pm&m=145463901630950&w=4
> > 
> > Also this is more of a brain dump rather than actual design description,
> > so there may be holes etc in it.  Please let me know if you can see any.
> > 
> > The problem at hand is that policy->rwsem needs to be held around *all*
> > operations in cpufreq_set_policy().  In particular, it cannot be dropped
> > around invocations of __cpufreq_governor() with the event arg equal to
> > _EXIT as that leads to interesting races.
> > 
> > Unfortunately, we know that holding policy->rwsem in those places leads
> > to a deadlock with governor sysfs attributes removal in cpufreq_governor_exit().
> > 
> > Viresh attempted to fix this by avoiding to acquire policy->rwsem for governor
> > attributes access (as holding it is not necessary for them in principle).  That
> > was a nice try, but it turned out to be insufficient because of another deadlock
> > scenario uncovered by it.
> 
> Not really.
> 
> The other deadlock wasn't uncovered by it, its just that Shilpa tested
> directly after my patches and reported the issue. Later yesterday, she
> was hitting the exactly same issue on pm/linux-next as well (i.e.
> without my patches). And ofcourse Juri has also reported the same
> issue on linux-next few days back.

OK, fair enough.

> > Namely, since the ondemand governor's update_sampling_rate()
> > acquires the governor mutex (called dbs_data_mutex after my patches mentioned
> > above), it may deadlock with exactly the same piece of code in cpufreq_governor_exit()
> > in almost exactly the same way.
> 
> Right.
> 
> > To avoid that other deadlock, we'd either need to drop dbs_data_mutex from
> > update_sampling_rate(),
> 
> And my so called 'ugly' 8th patch tried to do just that :)
> 
> But as I also mentioned in reply to the update-util patchset of yours,
> its possible somewhat.

Yes, it should be possible and not even too difficult.

> > or drop it for the removal of the governor sysfs
> > attributes in cpufreq_governor_exit().  I don't think the former is an option
> > at least at this point, so it looks like we pretty much have to do the latter.
> > 
> > With that in mind, I'd start with the changes made by Viresh (maybe without the
> > first patch which really isn't essential here).
> 
> That was just to cleanup the macro mess a bit, nothing more. Over
> that, I think the first 7 patches can be picked as it is without any
> changes. Ofcourse they are required to be rebased over your 13
> patches, if those are going in first :)

Yes, please rebase.

Also please skip the first one that was moving min_sampling_rate around,
at least for now.

As I said, we may be moving other attributes in the opposite direction,
so two sets of macros may be necessary anyway.

> > That is, introduce a separate
> > kobject type for the governor attributes kobject and register that in
> > cpufreq_governor_init().  The show/store callbacks for that kobject type won't
> > acquire policy->rwsem so the first deadlock will be avoided.
> > 
> > But in addition to that, I'd drop dbs_data_mutex before the removal of governor
> > sysfs attributes.  That actually happens in two places, in cpufreq_governor_exit()
> > and in the error path of cpufreq_governor_init().
> > 
> > To that end, I'd move the locking from cpufreq_governor_dbs() to the functions
> > called by it.  That should be readily doable and they can do all of the
> > necessary checks themselves.  cpufreq_governor_dbs() would become a pure mux then,
> > but that's not such a big deal.
> > 
> > With that, cpufreq_governor_exit() may just drop the lock before it does the
> > final kobject_put().  The danger here is that the sysfs show/store callbacks of
> > the governor attributes kobject may see invalid dbs_data for a while, after the
> > lock has been dropped and before the kobject is deleted.  That may be addressed
> > by checking, for example, the presence of the dbs_data's "tuners" pointer in those
> > callbacks.  If it is NULL, they can simply return -EAGAIN or similar.
> 
> So you mean something like this (consider only !governor_per_policy
> case with ondemand governor for now):
> 
> exit()
> {
>        lock-dbs_data_mutex;
>        ...
>        dbs_data->tuners = NULL; //so that sysfs files can return early
>        dbs_governor->gdbs_data = NULL; //For !governor_per_policy case
>        unlock-dbs_data_mutex;
> 
>        /*
>         * Problem: Who is stopping us to set ondemand as governor for
>         * another policy, which can try create a kobject which will
>         * try to create sysfs directory at the same path ?
>         *
>         * Though another field in dbs_governor can be used to fix this
>         * I think, which needs to block the other INIT operation.
>         */
>         
>        kobject_put(dbs_data->kobj); //This should wait for all sysfs operations to end.
> 
>        kfree(dbs_data);
> }
> 
> And the sysfs operations show/store need to take dbs_data_mutex() for
> their entire operations.
> 
> ??

Yes, roughly.

But it shouldn't be necessary after all, because dropping the mutex from
update_sampling_rate() looks easier than I thought previously.

Thanks,
Rafael