On Fri, Jun 11, 2010 at 09:46:49PM -0700, Linus Torvalds wrote: > On Fri, Jun 11, 2010 at 9:38 PM, Nick Piggin <npiggin@xxxxxxx> wrote: > > > > In fairness I added a lot more comments and a bit of whitespace, > > accounting for about half of those lines added. > > Yeah, ok. As mentioned, I can certainly go either way. > > > I wouldn't mind a list macro to reset. I'd prefer the name > > match better with the iterator macro though. > > list_safe_reset_next()? > > Yeah, it needs to be at least "list_" something. So > "list_safe_reset_next()" sounds as good as anything else. I wouldn't > expect it to be very common. I could go either way as well. This is what the list macro looks like. I almost dislike it because it is a rather special thing to be doing. OTOH it gives a pattern to grep for abuses. Al, ping? -- 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@xxxxxxx> --- fs/dcache.c | 2 ++ fs/super.c | 6 ++++++ 2 files changed, 8 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_for_each_entry_safe 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
