linux-kernel - Re: [PATCH RFC tip/core/rcu 24/28] rcu: Introduce bulk reference count

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Date:	Thu, 3 Nov 2011 13:19:12 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	patches@...aro.org
Subject: Re: [PATCH RFC tip/core/rcu 24/28] rcu: Introduce bulk reference
 count

On Thu, Nov 03, 2011 at 06:34:35AM -0700, Paul E. McKenney wrote:
> On Wed, Nov 02, 2011 at 09:34:41PM -0700, Josh Triplett wrote:
> > On Wed, Nov 02, 2011 at 01:30:45PM -0700, Paul E. McKenney wrote:
> > > The RCU implementations, including SRCU, are designed to be used in a
> > > lock-like fashion, so that the read-side lock and unlock primitives must
> > > execute in the same context for any given read-side critical section.
> > > This constraint is enforced by lockdep-RCU.  However, there is a need for
> > > something that acts more like a reference count than a lock, in order
> > > to allow (for example) the reference to be acquired within the context
> > > of an exception, while that same reference is released in the context of
> > > the task that encountered the exception.  The cost of this capability is
> > > that the read-side operations incur the overhead of disabling interrupts.
> > > Some optimization is possible, and will be carried out if warranted.
> > > 
> > > Note that although the current implementation allows a given reference to
> > > be acquired by one task and then released by another, all known possible
> > > implementations that allow this have scalability problems.  Therefore,
> > > a given reference must be released by the same task that acquired it,
> > > though perhaps from an interrupt or exception handler running within
> > > that task's context.
> > 
> > This new bulkref API seems in dire need of documentation. :)
> > 
> > > --- a/include/linux/srcu.h
> > > +++ b/include/linux/srcu.h
> > > @@ -181,4 +181,54 @@ static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
> > >  	__srcu_read_unlock(sp, idx);
> > >  }
> > >  
> > > +/* Definitions for bulkref_t, currently defined in terms of SRCU. */
> > > +
> > > +typedef struct srcu_struct bulkref_t;
> > > +int init_srcu_struct_fields(struct srcu_struct *sp);
> > > +
> > > +static inline int init_bulkref(bulkref_t *brp)
> > > +{
> > > +	return init_srcu_struct_fields(brp);
> > > +}
> > 
> > Why can't this call init_srcu_struct and avoid the need to use the
> > previously unexported internal function?
> 
> Seems reasonable now that you mention it.  ;-)

Except that doing so results in lockdep initialization that cannot be
used.  :-(

							Thanx, Paul

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