linux-kernel - Re: [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use lockless

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Message-ID: <20151029000030.GB4122@linux.vnet.ibm.com>
Date:	Wed, 28 Oct 2015 17:00:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Patrick Marlier <patrick.marlier@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Tejun Heo <tj@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	pranith kumar <bobby.prani@...il.com>
Subject: Re: [PATCH tip/core/rcu 11/13] rculist: Make list_entry_rcu() use
 lockless_dereference()

On Wed, Oct 28, 2015 at 09:35:42PM +0100, Patrick Marlier wrote:
> 
> 
> On 10/28/2015 09:33 AM, Ingo Molnar wrote:
> >
> >* Tejun Heo <tj@...nel.org> wrote:
> >
> >>Subject: writeback: don't use list_entry_rcu() for pointer offsetting in bdi_split_work_to_wbs()
> >>
> >>bdi_split_work_to_wbs() uses list_for_each_entry_rcu_continue() to
> >>walk @bdi->wb_list.  To set up the initial iteration condition, it
> >>uses list_entry_rcu() to calculate the entry pointer corresponding to
> >>the list head; however, this isn't an actual RCU dereference and using
> >>list_entry_rcu() for it ended up breaking a proposed list_entry_rcu()
> >>change because it was feeding an non-lvalue pointer into the macro.
> >>
> >>Don't use the RCU variant for simple pointer offsetting.  Use
> >>list_entry() instead.
> >>
> >>Signed-off-by: Tejun Heo <tj@...nel.org>
> >>---
> >>  fs/fs-writeback.c |    4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> >>index 29e4599..7378169 100644
> >>--- a/fs/fs-writeback.c
> >>+++ b/fs/fs-writeback.c
> >>@@ -779,8 +779,8 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
> >>  				  bool skip_if_busy)
> >>  {
> >>  	struct bdi_writeback *last_wb = NULL;
> >>-	struct bdi_writeback *wb = list_entry_rcu(&bdi->wb_list,
> >>-						struct bdi_writeback, bdi_node);
> >>+	struct bdi_writeback *wb = list_entry(&bdi->wb_list,
> >>+					      struct bdi_writeback, bdi_node);
> >>
> >>  	might_sleep();
> >
> >Any objections against me applying this fix to tip:core/rcu so that I can push the
> >recent RCU changes towards linux-next without triggering a build failure?
> 
> No objection on my side but probably you are waiting for an ack from
> somebody else.

I am guessing that he was asking Tejun, but just for the record, I am
OK with it as well:

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

							Thanx, Paul

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