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

Nick Piggin
2010-06-29T14:56:24Z

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@suse.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