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Subject: Re: [RFC,PATCH 14/14] utrace core
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>,
       Ananth Mavinakayanahalli <ananth@in.ibm.com>,
       Christoph Hellwig <hch@infradead.org>,
       "Frank Ch. Eigler" <fche@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       Roland McGrath <roland@redhat.com>, linux-kernel@vger.kernel.org,
       utrace-devel@redhat.com
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Date: Tue, 08 Dec 2009 16:29:23 +0100
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On Tue, 2009-12-08 at 16:04 +0100, Oleg Nesterov wrote:
> > > > > +       /*
> > > > > +        * In theory spin_lock() doesn't imply rcu_read_lock().
> > > > > +        * Once we clear ->utrace_flags this task_struct can go away
> > > > > +        * because tracehook_prepare_release_task() path does not take
> > > > > +        * utrace->lock when ->utrace_flags == 0.
> > > > > +        */
> > > > > +       rcu_read_lock();
> > > > > +       task->utrace_flags = flags;
> > > > > +       spin_unlock(&utrace->lock);
> > > > > +       rcu_read_unlock();
> > > >
> > > > yuck!
> > > >
> > > > why not simply keep a task reference over the utrace_reset call?
> > >
> > > Yes, we could use get_task_struct() instead. Not sure this would
> > > be more clean, though.
> >
> > For one it would allow getting rid of that insane assymetric locking.
> 
> Well, this is subjective, but I don't agree that
> 
>         get_task_struct(task);
>         task->utrace_flags = flags;
>         spin_unlock(&utrace->lock);
>         put_task_struct(task);
> 
> looks better. 

No, what I mean by assymetric locking is that utrace_reset() and
utrace_reap() drop the utrace->lock where their caller acquired it,
resulting in non-obvious like:

utrace_control()
{

  ...
  spin_lock(&utrace->lock);

  ...

  if (reset)
    utrace_reset(utrace);
  else
    spin_unlock(&utrace->lock);
}

If you take a task ref you can write the much saner:

utrace_control()
{
  ...
  spin_lock(&utrace->lock);
  ...
  if (reset)
    utrace_reset(utrace);

  spin_unlock(&utrace->lock);
}