Re: [KVM PATCH v3 1/3] KVM: fix race in irq_routing logic

[Paul E. McKenney]

On Tue, Oct 27, 2009 at 04:02:37PM +0200, Gleb Natapov wrote:
> On Tue, Oct 27, 2009 at 09:39:03AM -0400, Gregory Haskins wrote:

[ . . . ]

> > standard RCU RSCS, which is what SRCU is designed for.  So rather than
> > inventing an awkward two-phased stack based solution, it's better to
> > reuse the provided tools, IMO.
> > 
> > To flip it around:  Is there any reason why an SRCU would not work here,
> > and thus we were forced to use something like the stack-copy approach?
> > 
> If SRCU has no disadvantage comparing to RCU why not use it always? :)

The disadvantages of SRCU compared to RCU include the following:

1.	SRCU requires that the return value of srcu_read_lock()
	be fed into srcu_read_unlock().  This is usually not a problem,
	but can be painful if there are multiple levels of function
	call separating the two.

2.	SRCU's grace periods are about 4x slower than those of RCU.
	And they also don't scale all that well with extremely large
	numbers of CPUs (but this can be fixed when/if it becomes a
	real problem).

3.	SRCU's read-side primitives are also significantly slower than
	those of RCU.

4.	SRCU does not have a call_srcu().  One could be provided, but
	its semantics would be a bit strange due to the need to limit
	the number of callbacks, given that general blocking is
	permitted in SRCU read-side critical sections.  (And it would
	take some doing to convince me to supply an SRCU!)

5.	The current SRCU has no reasonable way to implement read-side
	priority boosting, as there is no record of which task
	is read-holding which SRCU.

Hey, you asked!  ;-)

							Thanx, Paul
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