linux-kernel - [tip:core/rcu] fs/writeback, rcu: Don't use list_entry_rcu() for pointer offsetting in bdi_split_work_to

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Date:	Wed, 28 Oct 2015 13:58:16 -0700
From:	tip-bot for Tejun Heo <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	edumazet@...gle.com, dipankar@...ibm.com,
	torvalds@...ux-foundation.org, jiangshanlai@...il.com,
	bobby.prani@...il.com, rostedt@...dmis.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	mathieu.desnoyers@...icios.com, dhowells@...hat.com, hpa@...or.com,
	dvhart@...ux.intel.com, tj@...nel.org, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de, josh@...htriplett.org, fweisbec@...il.com,
	oleg@...hat.com, patrick.marlier@...il.com
Subject: [tip:core/rcu] fs/writeback, rcu: Don't use list_entry_rcu()
  for pointer offsetting in bdi_split_work_to_wbs()

Commit-ID:  b33e18f61bd18227a456016a77b1a968f5bc1d65
Gitweb:     http://git.kernel.org/tip/b33e18f61bd18227a456016a77b1a968f5bc1d65
Author:     Tejun Heo <tj@...nel.org>
AuthorDate: Tue, 27 Oct 2015 14:19:39 +0900
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 28 Oct 2015 13:17:30 +0100

fs/writeback, rcu: 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.

Reported-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Darren Hart <dvhart@...ux.intel.com>
Cc: David Howells <dhowells@...hat.com>
Cc: Dipankar Sarma <dipankar@...ibm.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Patrick Marlier <patrick.marlier@...il.com>
Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: pranith kumar <bobby.prani@...il.com>
Link: http://lkml.kernel.org/r/20151027051939.GA19355@mtj.duckdns.org
Signed-off-by: Ingo Molnar <mingo@...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();
 restart:
--
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