Re: [PATCH] writeback_inodes can race with unmount

["Mika Penttilä" <mika.penttila@kolumbus.fi>]

Chris Mason wrote:

>There's a small window where the filesystem can be unmounted during
>writeback_inodes.  The end result is the iput done by sync_sb_inodes
>could be done after the FS put_super and and the super has been removed
>from all lists.
>  
>

Why don't we have the same race in the sync() path as well? Moving the 
locking to sync_sb_inodes() itself would fix it also.

>The fix is to hold the s_umount sem during sync_sb_inodes to make sure
>the FS doesn't get unmounted.
>
>Index: linux.t/fs/fs-writeback.c
>===================================================================
>--- linux.t.orig/fs/fs-writeback.c	2004-06-08 14:45:49.000000000 -0400
>+++ linux.t/fs/fs-writeback.c	2004-06-08 14:47:58.000000000 -0400
>@@ -360,9 +360,18 @@ restart:
> 	sb = sb_entry(super_blocks.prev);
> 	for (; sb != sb_entry(&super_blocks); sb = sb_entry(sb->s_list.prev)) {
> 		if (!list_empty(&sb->s_dirty) || !list_empty(&sb->s_io)) {
>+			/* we're making our own get_super here */
> 			sb->s_count++;
> 			spin_unlock(&sb_lock);
>-			sync_sb_inodes(sb, wbc);
>+			/* if we can't get the readlock, there's no sense in
>+			 * waiting around, most of the time the FS is going
>+			 * to be unmounted by the time it is released
>+			 */
>+			if (down_read_trylock(&sb->s_umount)) {
>+				if (sb->s_root)
>+					sync_sb_inodes(sb, wbc);
>+				up_read(&sb->s_umount);
>+			}
> 			spin_lock(&sb_lock);
> 			if (__put_super(sb))
> 				goto restart;
>  
>

--Mika