linux-kernel - Re: [PATCH 1/2] rcu: introduce list_last_or_null

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Message-ID: <20150528170542.3b3a7b48@gandalf.local.home>
Date:	Thu, 28 May 2015 17:05:42 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dan Streetman <ddstreet@...e.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rcu: introduce list_last_or_null_rcu

On Thu, 28 May 2015 16:35:27 -0400
Dan Streetman <ddstreet@...e.org> wrote:

> Add list_last_or_null_rcu(), to simplify getting the last entry from a
> rcu-protected list.  The standard list_last_entry() can't be used as it
> is not rcu-protected; the list may be modified concurrently.  And the
> ->prev pointer can't be used, as only the ->next pointers are protected
> by rcu.
> 
> This simply iterates forward through the entire list, to get to the last
> entry.  If the list is empty, it returns NULL.

May I asked what this would be used for? It seems awfully inefficient
in its implementation. What use cases would this be for? I hate to add
something like this as a generic function which would encourage people
to use it. Iterating over an entire list to find the last element is
just nasty.

-- Steve

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