linux-kernel - [PATCH v2] drivers/md: use proper rcu accessor

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Message-ID: <1416764069.17888.36.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sun, 23 Nov 2014 09:34:29 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Pranith Kumar <bobby.prani@...il.com>,
	Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.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>, dvhart@...ux.intel.com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: [PATCH v2] drivers/md: use proper rcu accessor

From: Eric Dumazet <edumazet@...gle.com>

rcu_dereference() should be used in sections protected by rcu_read_lock.

For writers, holding some kind of mutex or lock,
rcu_dereference_protected() is the way to go, adding explicit lockdep
bits.

In __unbind(), we are the last user of this mapped device, so can use
the constant '1' instead of a lockdep_is_held(), not consistent with
other uses of rcu_dereference_protected() which use md->suspend_lock
mutex.

Reported-by: Kirill A. Shutemov <kirill@...temov.name>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Fixes: 33423974bfc1 ("dm: Use rcu_dereference() for accessing rcu pointer")
Cc: Pranith Kumar <bobby.prani@...il.com>
Cc: Mike Snitzer <snitzer@...hat.com>
---

v2: changed all buggy rcu_dereference()
    BTW, having a mutex named suspend_lock is ugly, IMO.

 drivers/md/dm.c |   14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index a0ece87ad426..5919d933bce9 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2335,7 +2335,8 @@ static struct dm_table *__bind(struct mapped_device *md, struct dm_table *t,
 
 	merge_is_optional = dm_table_merge_is_optional(t);
 
-	old_map = rcu_dereference(md->map);
+	old_map = rcu_dereference_protected(md->map,
+					    lockdep_is_held(&md->suspend_lock));
 	rcu_assign_pointer(md->map, t);
 	md->immutable_target_type = dm_table_get_immutable_target_type(t);
 
@@ -2355,7 +2356,7 @@ static struct dm_table *__bind(struct mapped_device *md, struct dm_table *t,
  */
 static struct dm_table *__unbind(struct mapped_device *md)
 {
-	struct dm_table *map = rcu_dereference(md->map);
+	struct dm_table *map = rcu_dereference_protected(md->map, 1);
 
 	if (!map)
 		return NULL;
@@ -2850,7 +2851,8 @@ retry:
 		goto retry;
 	}
 
-	map = rcu_dereference(md->map);
+	map = rcu_dereference_protected(md->map,
+					lockdep_is_held(&md->suspend_lock));
 
 	r = __dm_suspend(md, map, suspend_flags, TASK_INTERRUPTIBLE);
 	if (r)
@@ -2908,7 +2910,8 @@ retry:
 		goto retry;
 	}
 
-	map = rcu_dereference(md->map);
+	map = rcu_dereference_protected(md->map,
+					lockdep_is_held(&md->suspend_lock));
 	if (!map || !dm_table_get_size(map))
 		goto out;
 
@@ -2943,7 +2946,8 @@ static void __dm_internal_suspend(struct mapped_device *md, unsigned suspend_fla
 		return; /* nest suspend */
 	}
 
-	map = rcu_dereference(md->map);
+	map = rcu_dereference_protected(md->map,
+					lockdep_is_held(&md->suspend_lock));
 
 	/*
 	 * Using TASK_UNINTERRUPTIBLE because only NOFLUSH internal suspend is


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