| Re: [PATCH][usercr]: Ghost tasks must be detached [message #41736 is a reply to message #41620] |
Wed, 16 February 2011 20:10   |
Sukadev Bhattiprolu
Messages: 413
Registered: August 2006
|
Senior Member |
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Oren Laadan [orenl@cs.columbia.edu] wrote:
| So instead, we can call __wake_up_parent() from exit_checkpoint()
| if indeed we are already reaped there:
|
| exit_checkpoint()
| {
| ...
| if (current->flags & PF_RESTARTING) {
| ...
| /* either zombie or reaped ghost/dead */
| if (current->exit_state = EXIT_DEAD)
| __wake_up_parent(...); /* probably need lock */
| ...
| }
| ...
| }
|
| and to avoid userspace misuse, disallow non-thread-group-leader ghosts.
|
| ?
Well, I don't see a problem as such, but notice one inconsistency.
By the time the ghost task calls exit_checkpoint() it would have
gone through release_task()/__exit_signal()/__unhash_process() so
it is no longer on the parent's ->children list. We will be accessing
the task's ->parent pointer after this.
I am looking to see if anything prevents the parent from itself going
through release_task(), after the child does the release_task() but before
the child does the exit_checkpoint().
In 2.6.38, I don't see specifically where a task's ->parent pointer is
invalidated. The task->parent and task->parent->signal are freed in the
final __put_task_struct(). So its probably safe to access them, even if the
parent itself is exiting and has gone through release_task().
But in 2.6.32 i.e RHEL5, tsk->signal is set to NULL in __exit_signal().
So, I am trying to rule out the following scenario:
Child (may not be a ghost) Parent
------------------------- ------
- exit_notify(): is EXIT_DEAD
- release_task():
- drops task_list_lock
- itself proceeds to exit.
- enters release_task()
- sets own->signal = NULL
(in 2.6.32, __exit_signal())
- enters exit_checkpoint()
- __wake_up_parent()
access parents->signal NULL ptr
Not sure if holding task_list_lock here is needed or will help.
Sukadev
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