Re: [PATCH 5/6] Makes procs file writable to move all threads bytgid at once

[Daniel Lezcano]

Li Zefan wrote:
> > > > Hi Ben,
> > > >
> > > > The current code (with or without your patch) may lead to an error
> > > > because the fork hook can fail and the exit hook is called in all the
> > > > cases making the fork / exit asymmetric.
> > > >
> > > >     
> > > >         
> > > The _current_ code won't lead to this error, because the fork hook
> > > can't fail.
> > >   
> > >       
> > Right, as no subsystem is using both hooks right now, the bug is never
> > triggered and the current code won't lead to an error.
> > But from my POV, there is a bug hidden in a corner waiting for a
> > subsystem to make use of the fail-able fork / exit :)
> >
> >     
>
> Actually the freezer subsystem is using the fork hook, but it doesn't
> need to be able to fail it.
>
> I don't think we can claim this a bug. If there is a new subsystem
> that needs fail-able fork hook, it has to extent the hook interface
> and adjust the code to meet its needs.
>
> We always adjust our code to meet new needs, don't we?
>   
Sure.

> > > > I will take the usual example with a cgroup with a counter of tasks, in
> > > > the fork hook it increments the counter, in the exit hook it decrements
> > > > the counter. There is one process in the cgroup, hence the counter value
> > > > is 1. Now this process forks and the fork hook fails before the task
> > > > counter is incremented to 2, this is not detected in copy process
> > > > function because the cgroup_fork_callbacks does not return an error, so
> > > > the process will be forked without error and when the process will exits
> > > > the counter will be decremented reaching 0 instead of 1.
> > > >
> > > > IMO, the correct fix should be to make the fork hook to return an error
> > > > and have the cgroup to call the exit method of the subsystem where the
> > > > fork hook was called. For example, there are 10 subsystems using the
> > > > fork / exit hooks, when the a process forks, the fork callbacks is
> > > > called for these subsystems but, let's say, the 3rd fails. So we undo,
> > > > by calling the exit hooks of the first two.
> > > >
> > > > I wrote a patchset to consolidate the hooks called in the cgroump for
> > > > fork and exit, and one of them does a rollback for the fork hook when an
> > > > error occurs. I added an attachment the patch as an example.
> > > >
> > > >     
> > > >         
> > > I'd like to see this patch sent with another patch that needs this
> > > fail-able fork() hook.
> > >
> > > Note this patch is not doing a _fix_, but does an extension. And
> > > for now, this extension is not needed.
> > >   
> > >       
> > I don't know, may be it could be interesting to implement that before
> > more subsystems make use of these hooks.
> > This is not critical, that can be sent later, separately from this
> > patchset of course.
> >
> >     
>
> We tend to remove code that is not used. For example, we may remove
> subsys->bind() interface, because no one is using it, though it has
> been there for years.
>
> So adding things that are not used is normally not good.
>   
Makes sense.

Thanks
 -- Daniel

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