super: wait for nascent superblocks

Recent patches experiment with making it possible to allocate a new
superblock before opening the relevant block device. Naturally this has
intricate side-effects that we get to learn about while developing this.

Superblock allocators such as sget{_fc}() return with s_umount of the
new superblock held and lock ordering currently requires that block
level locks such as bdev_lock and open_mutex rank above s_umount.

Before aca740cecb ("fs: open block device after superblock creation")
ordering was guaranteed to be correct as block devices were opened prior
to superblock allocation and thus s_umount wasn't held. But now s_umount
must be dropped before opening block devices to avoid locking
violations.

This has consequences. The main one being that iterators over
@super_blocks and @fs_supers that grab a temporary reference to the
superblock can now also grab s_umount before the caller has managed to
open block devices and called fill_super(). So whereas before such
iterators or concurrent mounts would have simply slept on s_umount until
SB_BORN was set or the superblock was discard due to initalization
failure they can now needlessly spin through sget{_fc}().

If the caller is sleeping on bdev_lock or open_mutex one caller waiting
on SB_BORN will always spin somewhere and potentially this can go on for
quite a while.

It should be possible to drop s_umount while allowing iterators to wait
on a nascent superblock to either be born or discarded. This patch
implements a wait_var_event() mechanism allowing iterators to sleep
until they are woken when the superblock is born or discarded.

This also allows us to avoid relooping through @fs_supers and
@super_blocks if a superblock isn't yet born or dying.

Link: aca740cecb ("fs: open block device after superblock creation")
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230818-vfs-super-fixes-v3-v3-3-9f0b1876e46b@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/fs.h

@@ -1095,6 +1095,7 @@ extern int send_sigurg(struct fown_struct *fown);
 #define SB_LAZYTIME     BIT(25)	/* Update the on-disk [acm]times lazily */
 
 /* These sb flags are internal to the kernel */
+#define SB_DYING        BIT(24)
 #define SB_SUBMOUNT     BIT(26)
 #define SB_FORCE        BIT(27)
 #define SB_NOSEC        BIT(28)