Re: [RFC,PATCH] RCUify single-thread case of clock_gettime()

[Hugh Dickins <hugh@veritas.com>]

On Mon, 31 Oct 2005, Paul E. McKenney wrote:
> 
> The attached patch uses RCU to avoid the need to acquire tasklist_lock
> in the single-thread case of clock_gettime().  Still acquires tasklist_lock
> when asking for the time of a (potentially multithreaded) process.
> 
> Experimental, has been touch-tested on x86 and POWER.  Requires RCU on
> task_struct.  Further more focused testing in progress.
> 
> Thoughts?  (Why?  Some off-list users want to be able to monitor CPU
> consumption of specific threads.  They need to do so quite frequently,
> so acquiring tasklist_lock is inappropriate.)

Not my area at all, but this looks really dodgy to me, Paul:
could you explain it further?

First off, I don't see what's "RCU" about it at all.  Essentially,
you're replacing read_lock(&tasklist_lock) by preempt_disable(),
but calling it by the fancier rcu_read_lock() alias.  I thought there
would need to be some more infrastructure to make this RCU and safe?

Hugh