On Fri 05-10-12 14:35:53, Fernando Luis Vázquez Cao wrote:
> The emergency thaw process uses iterate_super() which holds the
> sb->s_umount lock in read mode. The current thaw_super() code takes
> the sb->s_umount lock in write mode, hence leading to an instant
> deadlock.
>
> Use the unlocked version of thaw_super() to do the thawing and replace
> iterate_supers() with __iterate_supers() so that the unfreeze operation can
^^ iterate_supers_write()
> be performed with s_umount held as the locking rules for fsfreeze indicate.
>
> As a bonus, by using thaw_super(), which does not nest, instead of thaw_bdev()
> when can get rid of the ugly while loop.
>
> Jan Kara pointed out that with this approach we will leave the block devices
> frozen, but this is a problem we have had since the introduction of the
> superblock level API: if we thaw the filesystem using the superblock level API
> (be it through the thaw ioctl or emergency thaw) the bdev level freeze
> reference counter (bd_fsfreeze_count) will not be updated and even though
> subsequent calls to thaw_bdev() will decrease it it will never get back to 0
> (if thaw_super() returns an error, and it will when the superblock is unfrozen,
> thaw_bdev() will return without decreasing the counter). The solution I propose
> (and will be implementing in the followup patch "fsfreeze: freeze_super and
> thaw_bdev don't play well together") is letting bd_fsfreeze_count
> become zero when the superblock sitting on top of it is unfrozen, so that
> future calls to freeze_bdev() actually try to freeze the superblock.
>
> Cc: Josef Bacik <jbacik@xxxxxxxxxxxx>
> Cc: Eric Sandeen <sandeen@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Dave Chinner <dchinner@xxxxxxxxxx>
> Signed-off-by: Fernando Luis Vazquez Cao <fernando@xxxxxxxxxxxxx>
> ---
>
> diff -urNp linux-3.6.0-rc7-orig/fs/buffer.c linux-3.6.0-rc7/fs/buffer.c
> --- linux-3.6.0-rc7-orig/fs/buffer.c 2012-09-26 13:20:14.842365056 +0900
> +++ linux-3.6.0-rc7/fs/buffer.c 2012-09-26 15:02:22.630595704 +0900
> @@ -513,15 +513,28 @@ repeat:
>
> static void do_thaw_one(struct super_block *sb, void *unused)
> {
> - char b[BDEVNAME_SIZE];
> - while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
> - printk(KERN_WARNING "Emergency Thaw on %s\n",
> + int res;
> +
> + if (sb->s_bdev) {
> + char b[BDEVNAME_SIZE];
> + printk(KERN_WARNING "Emergency Thaw on %s.\n",
> bdevname(sb->s_bdev, b));
> + }
> +
> + /* We got here from __iterate_supers with the superblock lock taken
> + * so we can call the lockless version of thaw_super() safely. */
> + res = __thaw_super(sb);
> + /* If we are going to drop the final active reference call
> + * deactivate_locked_super to clean things up. In the general case
> + * we avoid calling deactivate_locked_super() because it would relase
> + * the superblock lock, which is __iterate_supers()'s job. */
> + if (!res && !atomic_add_unless(&sb->s_active, -1, 1))
> + deactivate_locked_super(sb);
This just looks wrong. When we *do* end up calling
deactivate_locked_super() we will return with sb unlocked which makes
iterate_supers_write() unlock already unlocked lock. What I would put here
is:
if (!res) {
deactivate_locked_super(sb);
/*
* We have to re-acquire s_umount because
* iterate_supers_write() will unlock it. It still holds
* passive reference so sb cannot be freed under us.
*/
down_write(&sb->s_umount);
}
Is there any problem with this I miss?
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
--
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
