Re: [rfc] "fair" rw spinlocks

Ingo Molnar
2009-11-30T17:46:52Z

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, 30 Nov 2009, Christoph Hellwig wrote:
> >
> > How long will this use be around? I've seen some slow progress toward
> > replacing most read side uses of the task list lock with RCU. While we
> > still have lots of read side users now I wonder when they'll go away.
>
> tasklist_lock is pretty nasty. I threw out "replace it with RCU"
> because it would be nice, but the data structures used are not just
> simple linked lists that we have RCU helpers for traversing.
>
> There are various real exclusion rules about things like
> 'tsk->exit_state' etc, which do not translate directly to RCU usage.
> Of course, _maybe_ all the places that care already take the thing for
> writing and would just automatically have exclusion anyway.
>
> So I'd love to see somebody try to do the conversion. To a first
> approximation, you probably could do
>
> - turn tasklist_lock into a spinlock
>
> - sed 's/write_lock_irq(&tasklist_lock)/spin_lock(&tasklist_lock)/g'
> sed 's/write_unlock_irq(&tasklist_lock)/spin_unlock(&tasklist_lock)/g'
>
> - sed 's/read_lock(&tasklist_lock)/rcu_read_lock()/g'
> sed 's/read_unlock(&tasklist_lock)/rcu_read_unlock()/g'
>
> - make all the task lists use the RCU versions of the list routines
>
> - free the task structure using RCU
>
> and you'd be _pretty_ close to a working system.

In -rt we've got that in essence, and it's indeed working fine (with a
few caveats). A few RCU conversions of tasklist_lock usage in that area
even trickled upstream, because the simple lock would hurt so much under
-rt.

Ingo