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    SubjectRe: [patch 02/52] fs: fix superblock iteration race
    This should actually be on it's way to Linus for .35, shouldn't it?

    On Thu, Jun 24, 2010 at 01:02:14PM +1000, npiggin@suse.de wrote:
    > 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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    ---end quoted text---


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