[Intel-gfx] [PATCH] drm: Include task->name and master status in debugfs clients info

[Chris Wilson]

On Mon, Sep 01, 2014 at 04:11:43PM +0200, David Herrmann wrote:
> Hi
> 
> On Sat, Aug 9, 2014 at 8:22 AM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > Showing who is the current master is useful for trying to decypher
> > errors when trying to acquire master (e.g. a race with X taking over
> > from plymouth). By including the process name as well as the pid
> > simplifies the task of grabbing enough information remotely at the point
> > of error.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/drm_info.c | 17 ++++++++++++-----
> >  1 file changed, 12 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_info.c b/drivers/gpu/drm/drm_info.c
> > index 15ec9f4..d813430 100644
> > --- a/drivers/gpu/drm/drm_info.c
> > +++ b/drivers/gpu/drm/drm_info.c
> > @@ -184,14 +184,21 @@ int drm_clients_info(struct seq_file *m, void *data)
> >         struct drm_file *priv;
> >
> >         mutex_lock(&dev->struct_mutex);
> > -       seq_printf(m, "a dev    pid    uid      magic\n\n");
> > -       list_for_each_entry(priv, &dev->filelist, lhead) {
> > -               seq_printf(m, "%c %3d %5d %5d %10u\n",
> > -                          priv->authenticated ? 'y' : 'n',
> > -                          priv->minor->index,
> > +       seq_printf(m, "                       pid dev master a   uid      magic\n");
> 
> Maybe mention "task" here? Or is this supposed to be a logical part of "pid"?

Yeah, I was thinking that comm/pid were essentially the same column. Top
uses command which would be reasonable to reuse.
 
> > +       list_for_each_entry_reverse(priv, &dev->filelist, lhead) {
> 
> No idea why you do backwards traversal, but ok..

Clients are added youngest-first. So traversing backwards gives
  kernel
  X/display manager
  clients
 
> > +               struct task_struct *task;
> > +
> > +               rcu_read_lock();
> 
> What's that rcu-lock for? task->comm is pre-allocated, priv->pid is
> static, kuid... no idea?

Cargo-culting the locking from the discussion with Tetsuo:

commit 3ec2f427e6f82b9b8f9b18dd2c758b864385df39
Author: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
Date:   Fri Jan 3 20:42:18 2014 +0900

    drm/i915: Fix refcount leak and possible NULL pointerdereference.
    
    Since get_pid_task() grabs a reference on the task_struct, we have to drop the
    refcount after reading that task's comm name. Use pid_task() with RCU instead.
    
    Also, avoid directly reading like pid_task()->comm because
    pid_task() will return NULL if the task have already exit()ed.
    
    This patch fixes both problems.


> Anyway, patch looks good otherwise. Especially task->comm sounds
> really handy in that list.

It was. Thanks for the feedback, will add the extra header and comment.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre