Re: [RFC PATCH 0/3] page count lock for simpler put_page

[Johannes Weiner <jweiner@redhat.com>]

On Fri, Aug 12, 2011 at 11:13:06AM -0700, Paul E. McKenney wrote:
> On Fri, Aug 12, 2011 at 07:52:06PM +0200, Johannes Weiner wrote:
> > On Fri, Aug 12, 2011 at 07:08:23PM +0200, Andrea Arcangeli wrote:
> > > On Fri, Aug 12, 2011 at 06:57:49PM +0200, Johannes Weiner wrote:
> > > > I understand you want to be careful with the promises you make in the
> > > > API.  How about not even exposing the check for whether a grace period
> > > > elapsed, but instead provide a specialized synchronize_rcu()?
> > > > 
> > > > Something like
> > > > 
> > > > 	void synchronize_rcu_with(rcu_time_t time)
> > > > 
> > > > that only promises all readers from the specified time are finished.
> > > > 
> > > > [ And synchronize_rcu() would be equivalent to
> > > >   synchronize_rcu_with(rcu_current_time()) if I am not mistaken. ]
> > > > 
> > > > Then you wouldn't need to worry about how the return value of
> > > > rcu_cookie_gp_elapsed() might be interpreted, could freely implement
> > > > it equal to synchronize_rcu() on TINY_RCU, the false positives with
> > > > small cookies would not be about correctness but merely performance.
> > > > 
> > > > And it should still be all that which the THP case requires.
> > > > 
> > > > Would that work?
> > > 
> > > rcu_time_t would still be an unsigned long long like I suggested?
> > 
> > Do we even need to make this fixed?  It can be unsigned long long for
> > now, but I could imagine leaving it up to the user depending how much
> > space she is able/willing to invest to save time:
> > 
> > 	void synchronize_rcu_with(unsigned long time, unsigned int bits)
> > 	{
> > 		if (generation_counter & ((1 << bits) - 1) == time)
> > 			synchronize_rcu();
> > 	}
> 
> This is indeed more convenient for this particular use case, but suppose
> that the caller instead wanted to use call_rcu()?

I don't quite understand.  call_rcu() will always schedule the
callbacks for execution after a grace period.  So the only use case I
can see--executing the callback ASAP as the required grace period has
already elapsed--would still require an extra argument to call_rcu()
for it to properly schedule the callback, no?  I.e.

	call_rcu_after(head, func, generation)

What am I missing that would make the existing call_rcu() useful in
combination with rcu_cookie_gp_elapsed()?

> The API I am currently proposing allows either synchronize_rcu() or
> call_rcu() to be used.  In addition, it allows alternative
> algorithms, for example:
> 
> 	rcu_get_gp_cookie(&wherever);
> 
> 	...
> 
> 	if (rcu_cookie_gp_elapsed(&wherever))
> 		p = old_pointer;  /* now safe to re-use. */
> 	else
> 		p = kmalloc( ... );  /* can't re-use, so get new memory. */

I have to admit that I am not imaginative enough right now to put this
in a real life scenario.  But it does look more flexible.

Though it must be made clear that it may never return true, so
anything essential (like _freeing_ old memory) may never rely on it.

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