Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once

Benjamin Blum
2009-08-06T00:01:49Z

On Wed, Aug 5, 2009 at 9:42 AM, Louis Rilling<Louis.Rilling@kerlabs.com> wrote:
> On 05/08/09  9:11 -0700, Paul Menage wrote:
>> On Wed, Aug 5, 2009 at 3:20 AM, Louis Rilling<Louis.Rilling@kerlabs.com> wrote:
>> >
>> > The downside of this is teaching lockdep about this recursive locking. Not that
>> > simple actually...
>>
>> Don't we just give each thread's lock its own lock class? That's what
>> we did for the cgroup hierarchy_mutex.
>
> Given that lock classes must be static and that lockdep only supports a limited
> lock depth, this is an issue for processes having many threads.
>
>>
>> > so that such cases are currently handled using a higher-level
>> > lock that prevents races in locking the whole chain (there was one such example
>> > for locking all vmas with KVM). IIUC, the intent here is to avoid such
>> > higher-level lock.
>>
>> cgroup_mutex already fulfills the role of the higher-level lock.
>
> If so (that is, here cgroup_mutex is taken before write-locking all threads'
> rw_sem), then enhancing rwsem's interface in a similar way to the
> spin_lock_nest_lock() interface could do it. There will still be an issue with
> many threads and lockdep limited lock depth though.

If we make the locks per-thread, then we can use plain mutexes instead
of rwsems since the only reader will ever be the owning thread itself,
and we can use mutex_lock_nested.

>
> Added Peter in CC.
>
> Louis
>
> --
> Dr Louis Rilling                        Kerlabs
> Skype: louis.rilling                    Batiment Germanium
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