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Commit 57439f87 authored by npiggin@suse.de's avatar npiggin@suse.de Committed by Linus Torvalds
Browse files

fs: fix superblock iteration race 



list_for_each_entry_safe is not suitable to protect against concurrent
modification of the list. 6754af64 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: default avatarNick Piggin <npiggin@suse.de>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Frank Mayhar <fmayhar@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5904b3b8

fs/dcache.c

+2 −0
Original line number Diff line number Diff line
@@ -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? */

fs/super.c

+6 −0
Original line number Diff line number Diff line
@@ -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 super_block *, void *), void *arg) 
		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 work_struct *work) 
		}

		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);

include/linux/list.h

+15 −0
Original line number Diff line number Diff line
@@ -544,6 +544,21 @@ static inline void list_splice_tail_init(struct list_head *list, 
	     &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