linux-kernel - Re: [patch 02/52] fs: fix superblock iteration race

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Date:	Wed, 30 Jun 2010 00:56:17 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>, torvalds@...l.org,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [patch 02/52] fs: fix superblock iteration race

On Tue, Jun 29, 2010 at 09:02:14AM -0400, Christoph Hellwig wrote:
> This should actually be on it's way to Linus for .35, shouldn't it?

Yeah, I was waiting for Al to reappear, but I think this is
probably the nicest way to solve the problem. Linus?
--
fs: fix superblock iteration race

list_for_each_entry_safe is not suitable to protect against concurrent
modification of the list. 6754af6 introduced a race in sb walking.

list_for_each_entry can use the trick of pinning the current entry in
the list before we drop and retake the lock because it subsequently
follows cur->next. However list_for_each_entry_safe saves n=cur->next
for following before entering the loop body, so when the lock is
dropped, n may be deleted.

Signed-off-by: Nick Piggin <npiggin@...e.de>
---
 fs/dcache.c          |    2 ++
 fs/super.c           |    6 ++++++
 include/linux/list.h |   15 +++++++++++++++
 3 files changed, 23 insertions(+)

Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c
+++ linux-2.6/fs/dcache.c
@@ -590,6 +590,8 @@ static void prune_dcache(int count)
 			up_read(&sb->s_umount);
 		}
 		spin_lock(&sb_lock);
+		/* lock was dropped, must reset next */
+		list_safe_reset_next(sb, n, s_list);
 		count -= pruned;
 		__put_super(sb);
 		/* more work left to do? */
Index: linux-2.6/fs/super.c
===================================================================
--- linux-2.6.orig/fs/super.c
+++ linux-2.6/fs/super.c
@@ -374,6 +374,8 @@ void sync_supers(void)
 			up_read(&sb->s_umount);
 
 			spin_lock(&sb_lock);
+			/* lock was dropped, must reset next */
+			list_safe_reset_next(sb, n, s_list);
 			__put_super(sb);
 		}
 	}
@@ -405,6 +407,8 @@ void iterate_supers(void (*f)(struct sup
 		up_read(&sb->s_umount);
 
 		spin_lock(&sb_lock);
+		/* lock was dropped, must reset next */
+		list_safe_reset_next(sb, n, s_list);
 		__put_super(sb);
 	}
 	spin_unlock(&sb_lock);
@@ -585,6 +589,8 @@ static void do_emergency_remount(struct
 		}
 		up_write(&sb->s_umount);
 		spin_lock(&sb_lock);
+		/* lock was dropped, must reset next */
+		list_safe_reset_next(sb, n, s_list);
 		__put_super(sb);
 	}
 	spin_unlock(&sb_lock);
Index: linux-2.6/include/linux/list.h
===================================================================
--- linux-2.6.orig/include/linux/list.h
+++ linux-2.6/include/linux/list.h
@@ -544,6 +544,21 @@ static inline void list_splice_tail_init
 	     &pos->member != (head); 					\
 	     pos = n, n = list_entry(n->member.prev, typeof(*n), member))
 
+/**
+ * list_safe_reset_next - reset a stale list_for_each_entry_safe loop
+ * @pos:	the loop cursor used in the list_for_each_entry_safe loop
+ * @n:		temporary storage used in list_for_each_entry_safe
+ * @member:	the name of the list_struct within the struct.
+ *
+ * list_safe_reset_next is not safe to use in general if the list may be
+ * modified concurrently (eg. the lock is dropped in the loop body). An
+ * exception to this is if the cursor element (pos) is pinned in the list,
+ * and list_safe_reset_next is called after re-taking the lock and before
+ * completing the current iteration of the loop body.
+ */
+#define list_safe_reset_next(pos, n, member)				\
+	n = list_entry(pos->member.next, typeof(*pos), member)
+
 /*
  * Double linked lists with a single pointer list head.
  * Mostly useful for hash tables where the two pointer list head is
--
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