Re: [PATCH v2 3/4] super: wait for nascent superblocks

[Jan Kara <jack@suse.cz>]

On Fri 18-08-23 12:54:17, Christian Brauner wrote:
> 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 aca740cecbe5 ("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: aca740cecbe5 ("fs: open block device after superblock creation")
> Signed-off-by: Christian Brauner <brauner@kernel.org>

Looks mostly good to me. I've spotted only a couple of nits and one
possible memory ordering issue...

> @@ -86,6 +81,94 @@ static inline void super_unlock_shared(struct super_block *sb)
>  	super_unlock(sb, false);
>  }
>  
> +static inline bool wait_born(struct super_block *sb)
> +{
> +	unsigned int flags;
> +
> +	/*
> +	 * Pairs with smp_store_release() in super_wake() and ensures
> +	 * that we see SB_BORN or SB_DYING after we're woken.
> +	 */
> +	flags = smp_load_acquire(&sb->s_flags);
> +	return flags & (SB_BORN | SB_DYING);
> +}
> +
> +/**
> + * super_lock - wait for superblock to become ready

Perhaps expand this a bit to "wait for superblock to become ready and
lock it"

> + * @sb: superblock to wait for
> + * @excl: whether exclusive access is required
> + *
> + * If the superblock has neither passed through vfs_get_tree() or
> + * generic_shutdown_super() yet wait for it to happen. Either superblock
> + * creation will succeed and SB_BORN is set by vfs_get_tree() or we're
> + * woken and we'll see SB_DYING.
> + *
> + * The caller must have acquired a temporary reference on @sb->s_count.
> + *
> + * Return: This returns true if SB_BORN was set, false if SB_DYING was
> + *         set. The function acquires s_umount and returns with it held.
> + */
> +static bool super_lock(struct super_block *sb, bool excl)

Perhaps we can make the function __must_check? Because if you don't care
about the result you should be using __super_lock().

> +{
> +
> +	lockdep_assert_not_held(&sb->s_umount);
> +
> +relock:
> +	__super_lock(sb, excl);
> +
> +	/*
> +	 * Has gone through generic_shutdown_super() in the meantime.
> +	 * @sb->s_root is NULL and @sb->s_active is 0. No one needs to
> +	 * grab a reference to this. Tell them so.
> +	 */
> +	if (sb->s_flags & SB_DYING)
> +		return false;
> +
> +	/* Has called ->get_tree() successfully. */
> +	if (sb->s_flags & SB_BORN)
> +		return true;
> +
> +	super_unlock(sb, excl);
> +
> +	/* wait until the superblock is ready or dying */
> +	wait_var_event(&sb->s_flags, wait_born(sb));
> +
> +	/*
> +	 * Neither SB_BORN nor SB_DYING are ever unset so we never loop.
> +	 * Just reacquire @sb->s_umount for the caller.
> +	 */
> +	goto relock;
> +}
> +
> +/* wait and acquire read-side of @sb->s_umount */
> +static inline bool super_lock_shared(struct super_block *sb)
> +{
> +	return super_lock(sb, false);
> +}
> +
> +/* wait and acquire write-side of @sb->s_umount */
> +static inline bool super_lock_excl(struct super_block *sb)
> +{
> +	return super_lock(sb, true);
> +}
> +
> +/* wake waiters */
> +#define SUPER_WAKE_FLAGS (SB_BORN | SB_DYING)
> +static void super_wake(struct super_block *sb, unsigned int flag)
> +{
> +	unsigned int flags = sb->s_flags;
> +
> +	WARN_ON_ONCE((flag & ~SUPER_WAKE_FLAGS));
> +	WARN_ON_ONCE(hweight32(flag & SUPER_WAKE_FLAGS) > 1);

Maybe assert here that s_umount is held?

> +
> +	/*
> +	 * Pairs with smp_load_acquire() in super_lock() and
> +	 * ensures that @flag is set before we wake anyone.
> +	 */
> +	smp_store_release(&sb->s_flags, flags | flag);
> +	wake_up_var(&sb->s_flags);

As I'm thinking about it now, we may need at least a smp_rmb() between the
store and wake_up_var(). What I'm worried about is the following:

TASK1					TASK2
super_wake()				super_lock()
					  check s_flags, SB_BORN not set yet
  waitqueue_active() from wake_up_var()
    which got reordered by the CPU before
    smp_store_release(). This seems possible
    because release is a one-way permeable in
    this direction.
					  wait_var_event(..)
					    prepare_to_wait_event()
					    wait_born()
					      SB_BORN still not set => sleep
  smp_store_release() sets SB_BORN
  wake_up_var() does nothing because it thinks
    the waitqueue is empty.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR