Re: [PATCH 2/3] ftrace: use struct pid

[Dipankar Sarma <dipankar@in.ibm.com>]

On Thu, Dec 04, 2008 at 05:07:02AM -0800, Dave Hansen wrote:
> On Thu, 2008-12-04 at 04:56 -0800, Dave Hansen wrote:
> > On Thu, 2008-12-04 at 04:42 -0800, Eric W. Biederman wrote:
> > > 
> > > > +static void clear_ftrace_pid_task(struct pid **pid)
> > > > +{
> > > > +     struct task_struct *p;
> > > > +
> > >         rcu_read_lock();
> > > 
> > > > +     do_each_pid_task(*pid, PIDTYPE_PID, p) {
> > > > +             clear_tsk_trace_trace(p);
> > > > +     } while_each_pid_task(*pid, PIDTYPE_PID, p);
> > >         rcu_read_unlock()
> > > 
> > > > +     put_pid(*pid);
> > > > +
> > > > +     *pid = NULL;
> > > > +}
> > 
> > Could we get away with sticking the rcu_read_{un}lock() inside those
> > macros?  Those are going to get used in pretty high level code and we're
> > allowed to nest rcu_read_lock().  No danger of deadlocks or lock
> > inversions.
> 
> Why don't any of the other users of do_each_pid_task() use
> rcu_read_lock()?  They all seem to be under read_lock(&tasklist_lock)
> (except one is under a write lock of the same).

The pid hash list is protected by tasklist_lock, right ? If so,
holding read_lock(&tasklist_lock) will make this safe, you don't
need rcu_read_lock/unlock(). This isn't a lock-free reader.

Thanks
Dipankar