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From: Ben Blum on 4 Aug 2010 00:40
On Tue, Aug 03, 2010 at 08:44:01PM -0700, Paul Menage wrote:
> On Fri, Jul 30, 2010 at 4:57 PM, Ben Blum <bblum(a)andrew.cmu.edu> wrote:
> > + * The threadgroup_fork_lock prevents threads from forking with
> > + * CLONE_THREAD while held for writing. Use this for fork-sensitive
> > + * threadgroup-wide operations. It's taken for reading in fork.c in
> > + * copy_process().
> > + * Currently only needed write-side by cgroups.
> > + */
> > + struct rw_semaphore threadgroup_fork_lock;
> > +#endif
>
> I'm not sure how best to word this comment, but I'd prefer something like:
>
> "The threadgroup_fork_lock is taken in read mode during a CLONE_THREAD
> fork operation; taking it in write mode prevents the owning
> threadgroup from adding any new threads and thus allows you to
> synchronize against the addition of unseen threads when performing
> threadgroup-wide operations. New-process forks (without CLONE_THREAD)
> are not affected."

That sounds good.

> As far as the #ifdef mess goes, it's true that some people don't have
> CONFIG_CGROUPS defined. I'd imagine that these are likely to be
> embedded systems with a fairly small number of processes and threads
> per process. Are there really any such platforms where the cost of a
> single extra rwsem per process is going to make a difference either in
> terms of memory or lock contention? I think you should consider making
> these additions unconditional.

That's certainly an option, but I think it would be clean enough to put
static inline functions just under the signal_struct definition.
Thoughts?

>
> Paul
>

-- Ben
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From: Ben Blum on 6 Aug 2010 02:10
On Tue, Aug 03, 2010 at 09:34:22PM -0700, Paul Menage wrote:
> On Tue, Aug 3, 2010 at 9:33 PM, Ben Blum <bblum(a)andrew.cmu.edu> wrote:
> >> As far as the #ifdef mess goes, it's true that some people don't have
> >> CONFIG_CGROUPS defined. I'd imagine that these are likely to be
> >> embedded systems with a fairly small number of processes and threads
> >> per process. Are there really any such platforms where the cost of a
> >> single extra rwsem per process is going to make a difference either in
> >> terms of memory or lock contention? I think you should consider making
> >> these additions unconditional.
> >
> > That's certainly an option, but I think it would be clean enough to put
> > static inline functions just under the signal_struct definition.
>
> Either sounds fine to me. I suspect others have a stronger opinion.
>
> Paul
>

Any other votes? One set of static inline functions (I'd call them
threadgroup_fork_{read,write}_{un,}lock) or just remove the ifdefs
entirely? I'm inclined to go with the former.

-- Ben
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From: KAMEZAWA Hiroyuki on 6 Aug 2010 03:20
On Fri, 6 Aug 2010 02:02:24 -0400
Ben Blum <bblum(a)andrew.cmu.edu> wrote:

> On Tue, Aug 03, 2010 at 09:34:22PM -0700, Paul Menage wrote:
> > On Tue, Aug 3, 2010 at 9:33 PM, Ben Blum <bblum(a)andrew.cmu.edu> wrote:
> > >> As far as the #ifdef mess goes, it's true that some people don't have
> > >> CONFIG_CGROUPS defined. I'd imagine that these are likely to be
> > >> embedded systems with a fairly small number of processes and threads
> > >> per process. Are there really any such platforms where the cost of a
> > >> single extra rwsem per process is going to make a difference either in
> > >> terms of memory or lock contention? I think you should consider making
> > >> these additions unconditional.
> > >
> > > That's certainly an option, but I think it would be clean enough to put
> > > static inline functions just under the signal_struct definition.
> >
> > Either sounds fine to me. I suspect others have a stronger opinion.
> >
> > Paul
> >
>
> Any other votes? One set of static inline functions (I'd call them
> threadgroup_fork_{read,write}_{un,}lock) or just remove the ifdefs
> entirely? I'm inclined to go with the former.
>

I vote for the former. #ifdef can be easily removed if someone finds it useful
for other purpose...and static inline function is usual way.

Thanks,
-Kame

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