Re: [PATCH 2/2] posix-cpu-timers: use ->sighand instead of ->signal to check the task is alive

[Oleg Nesterov <oleg@redhat.com>]

On 02/04, Peter Zijlstra wrote:
>
> On Wed, 2009-02-04 at 00:17 +0100, Oleg Nesterov wrote:
> > Sadly, it is not trivial to audit kernel/posix-cpu-timers.c, but it really
> > abuses tasklist_lock. I believe it doesn't need this lock at all, but the
> > changes are not easy to test.
>
> It uses that to hold of task reaping so ->signal doesn't go away.

Yes sure, but ->siglock alone is enough (this was not true when this code
was written, as far as I know). It is not trivial to remove tasklist
completely, but some places are trivial.

> If we make ->signal refcountable, and rcu freed along with the tasks I
> think we can get away without tasklist_lock.

I think this is possible even without this change (which is good anyway).
But the problem is not only that ->signal can go away. For example,
posix_cpu_timer_set/posix_cpu_timer_schedule should not proceed if the
task was already released, even if it had the valid ->signal.

Oleg.