linux-kernel - [PATCH v2 2/2] aio: kill the misleading rcu read locks in ioctx_add_table() and kill

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Message-ID: <20140430141636.GC18539@redhat.com>
Date:	Wed, 30 Apr 2014 16:16:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Kent Overstreet <kmo@...erainc.com>,
	Al Viro <viro@...iv.linux.org.uk>, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] aio: kill the misleading rcu read locks in
	ioctx_add_table() and kill_ioctx()

ioctx_add_table() is the writer, it does not need rcu_read_lock() to
protect ->ioctx_table. It relies on mm->ioctx_lock and rcu locks just
add the confusion.

And it doesn't need rcu_dereference() by the same reason, it must see
any updates previously done under the same ->ioctx_lock. We could use
rcu_dereference_protected() but the patch uses rcu_dereference_raw(),
the function is simple enough.

The same for kill_ioctx(), although it does not update the pointer.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 fs/aio.c |   14 +++-----------
 1 files changed, 3 insertions(+), 11 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 3526c2b..a415870 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -554,8 +554,7 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 	struct aio_ring *ring;
 
 	spin_lock(&mm->ioctx_lock);
-	rcu_read_lock();
-	table = rcu_dereference(mm->ioctx_table);
+	table = rcu_dereference_raw(mm->ioctx_table);
 
 	while (1) {
 		if (table)
@@ -563,7 +562,6 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 				if (!table->table[i]) {
 					ctx->id = i;
 					table->table[i] = ctx;
-					rcu_read_unlock();
 					spin_unlock(&mm->ioctx_lock);
 
 					/* While kioctx setup is in progress,
@@ -577,8 +575,6 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 				}
 
 		new_nr = (table ? table->nr : 1) * 4;
-
-		rcu_read_unlock();
 		spin_unlock(&mm->ioctx_lock);
 
 		table = kzalloc(sizeof(*table) + sizeof(struct kioctx *) *
@@ -589,8 +585,7 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 		table->nr = new_nr;
 
 		spin_lock(&mm->ioctx_lock);
-		rcu_read_lock();
-		old = rcu_dereference(mm->ioctx_table);
+		old = rcu_dereference_raw(mm->ioctx_table);
 
 		if (!old) {
 			rcu_assign_pointer(mm->ioctx_table, table);
@@ -737,12 +732,9 @@ static int kill_ioctx(struct mm_struct *mm, struct kioctx *ctx,
 
 
 	spin_lock(&mm->ioctx_lock);
-	rcu_read_lock();
-	table = rcu_dereference(mm->ioctx_table);
-
+	table = rcu_dereference_raw(mm->ioctx_table);
 	WARN_ON(ctx != table->table[ctx->id]);
 	table->table[ctx->id] = NULL;
-	rcu_read_unlock();
 	spin_unlock(&mm->ioctx_lock);
 
 	/* percpu_ref_kill() will do the necessary call_rcu() */
-- 
1.5.5.1


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