Re: [rfc][patch] Avoid taking global tasklist_lock for single threadedprocess at getrusage()

[Oleg Nesterov <oleg@tv-sign.ru>]

Ravikiran G Thirumalai wrote:
>
> On Sun, Jan 08, 2006 at 02:49:31PM +0300, Oleg Nesterov wrote:
> > > +       } else
> > > +               /* See locking comments above */
> > > +               smp_rmb();
> >
> > This patch doesn't try to optimize ->sighand.siglock locking,
> > and I think this is right. But this also means we don't need
> > rmb() here. It was needed to protect against "another thread
> > just exited, cpu can read ->c* values before thread_group_empty()
> > without taking siglock" case, now it is not possible.
>
> Don't we still need rmb for the RUSAGE_SELF case? we do not take the
> siglock for rusage self and the non c* signal fields are written to
> at __exit_signal...

I think it is unneeded because RUSAGE_SELF case is "racy" anyway even
if we held both locks, task_struct->xxx counters can change at any
moment.

But may be you are right.

> What is wrong with optimizing by not taking the siglock in RUSAGE_BOTH
> and RUSAGE_CHILDREN?  I would like to add that in too unless  I am
> missing something and the optimization is incorrect.

We can't have contention on ->siglock when need_lock == 0, so why should
we optimize this case?

Oleg.