Re: [rfc] "fair" rw spinlocks

[Ingo Molnar <mingo@elte.hu>]

* 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