[RFC][2.6.8-rc1-mm1] perfctr inheritance locking issue

[Mikael Pettersson <mikpe@csd.uu.se>]

Andrew,

There is another locking problem with the per-process
performance counter inheritance changes I sent you.

I currently use task_lock(tsk) to synchronise accesses
to tsk->thread.perfctr, when that pointer could change.

The write_lock_irq(&tasklist_lock) in release_task() is
needed to prevent ->parent from changing while releasing the
child, but the parent's ->thread.perfctr must also be locked.
However, sched.h explicitly forbids holding task_lock()
simultaneously with write_lock_irq(&tasklist_lock). Ouch.

My options seem to boil down to one of the following:
1. Forget task_lock(), always take the tasklist_lock.
   This should work but would lock the task list briefly at
   operations like set_cpus_allowed(), and creating/deleting
   a task's perfctr state object. I don't like that.
2. Add a 'spinlock_t perfctr_lock;' to the thread_struct,
   next to the perfctr state pointer. This is much cleaner,
   but increases the size of the thread struct slightly.

I think I prefer option #2. Any objections to that?

/Mikael