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    SubjectRe: [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@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

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