Messages in this thread | | | Date | Mon, 30 Nov 2009 18:46:38 +0100 | From | Ingo Molnar <> | Subject | Re: [rfc] "fair" rw spinlocks |
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* 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
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