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

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Date:	Wed, 2 Nov 2011 21:34:41 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 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?

- Josh Triplett
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