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

[Andrew Morton <akpm@osdl.org>]

Mikael Pettersson <mikpe@csd.uu.se> wrote:
>
> 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.

That's ghastly.

 * Nests both inside and outside of read_lock(&tasklist_lock).
 * It must not be nested with write_lock_irq(&tasklist_lock),
 * neither inside nor outside.

Manfred, where did you discover the offending code?

> 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?

Would be better to just sort out the locking, then take task_lock() inside
tasklist_lock.  That was allegedly the rule in 2.4.