On Thu 17-08-23 12:47:43, 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 ock 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 potentially this can go on for
^^ and potentially?
> 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 should also allows us to avoid relooping through @fs_supers and
^^^ superfluous "should"
> @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@xxxxxxxxxx>
> ---
> fs/super.c | 146 +++++++++++++++++++++++++++++++++++++++++++++--------
> include/linux/fs.h | 1 +
> 2 files changed, 125 insertions(+), 22 deletions(-)
>
> diff --git a/fs/super.c b/fs/super.c
> index 878675921bdc..55bf495763d9 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -86,6 +86,89 @@ static inline void super_unlock_read(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 ensure
> + * 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_wait - wait for superblock to become ready
> + * @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: true if SB_BORN was set, false if SB_DYING was set.
The comment should mention that this acquires s_umount and returns with it
held. Also the name is a bit too generic for my taste and not expressing
the fact this is in fact a lock operation. Maybe something like
super_lock_wait_born()?
> + */
> +static bool super_wait(struct super_block *sb, bool excl)
> +{
> +
> + 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_wait_read(struct super_block *sb)
> +{
> + return super_wait(sb, false);
> +}
And I'd call this super_lock_read_wait_born().
> +
> +/* wait and acquire write-side of @sb->s_umount */
> +static inline bool super_wait_write(struct super_block *sb)
> +{
> + return super_wait(sb, true);
> +}
> +
> +/* wake waiters */
> +static void super_wake(struct super_block *sb, unsigned int flag)
> +{
> + unsigned int flags = sb->s_flags;
I kind of miss the point of this local variable but whatever...
> +
> + /*
> + * Pairs with smp_load_acquire() in super_wait() and ensure that
^^^ ensures
> + * we @flag is set before we wake anyone.
^^ the
> + */
> + smp_store_release(&sb->s_flags, flags | flag);
> + wake_up_var(&sb->s_flags);
> +}
> +
> /*
> * One thing we have to be careful of with a per-sb shrinker is that we don't
> * drop the last active reference to the superblock from within the shrinker.
> @@ -415,10 +498,12 @@ EXPORT_SYMBOL(deactivate_super);
> */
> static int grab_super(struct super_block *s) __releases(sb_lock)
> {
> + bool born;
> +
> s->s_count++;
> spin_unlock(&sb_lock);
> - super_lock_write(s);
> - if ((s->s_flags & SB_BORN) && atomic_inc_not_zero(&s->s_active)) {
> + born = super_wait_write(s);
> + if (born && atomic_inc_not_zero(&s->s_active)) {
> put_super(s);
> return 1;
> }
> @@ -557,6 +642,13 @@ void generic_shutdown_super(struct super_block *sb)
> /* should be initialized for __put_super_and_need_restart() */
> hlist_del_init(&sb->s_instances);
> spin_unlock(&sb_lock);
> + /*
> + * Broadcast to everyone that grabbed a temporary reference to this
> + * superblock before we removed it from @fs_supers that the superblock
> + * is dying. Every walker of @fs_supers outside of sget{_fc}() will now
> + * discard this superblock and treat it as dead.
> + */
> + super_wake(sb, SB_DYING);
> super_unlock_write(sb);
> if (sb->s_bdi != &noop_backing_dev_info) {
> if (sb->s_iflags & SB_I_PERSB_BDI)
> @@ -631,6 +723,11 @@ struct super_block *sget_fc(struct fs_context *fc,
> s->s_type = fc->fs_type;
> s->s_iflags |= fc->s_iflags;
> strscpy(s->s_id, s->s_type->name, sizeof(s->s_id));
> + /*
> + * Make the superblock visible on @super_blocks and @fs_supers.
> + * It's in a nascent state and users should wait on SB_BORN or
> + * SB_DYING to be set.
> + */
But now sget_fc() (and sget()) will be looping on superblocks with SB_DYING
set? Ah, that's solved by the next patch. OK.
> list_add_tail(&s->s_list, &super_blocks);
> hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
> spin_unlock(&sb_lock);
<snip>
> @@ -841,15 +942,14 @@ struct super_block *get_active_super(struct block_device *bdev)
> if (!bdev)
> return NULL;
>
> -restart:
> spin_lock(&sb_lock);
> list_for_each_entry(sb, &super_blocks, s_list) {
> if (hlist_unhashed(&sb->s_instances))
> continue;
> if (sb->s_bdev == bdev) {
> if (!grab_super(sb))
> - goto restart;
> - super_unlock_write(sb);
> + return NULL;
Let me check whether I understand the rationale of this change: We found
a matching sb and it's SB_DYING. Instead of waiting for it to die and retry
the search (to likely not find anything) we just return NULL right away to
save us some trouble.
> + super_unlock_write(sb);
^^^ whitespace damage here
> return sb;
> }
> }
<snip>
> @@ -1212,9 +1314,9 @@ EXPORT_SYMBOL(get_tree_keyed);
> */
> static bool lock_active_super(struct super_block *sb)
Another inconsistently named locking function after you've introduced your
locking helpers...
> {
> - super_lock_read(sb);
> - if (!sb->s_root ||
> - (sb->s_flags & (SB_ACTIVE | SB_BORN)) != (SB_ACTIVE | SB_BORN)) {
> + bool born = super_wait_read(sb);
> +
> + if (!born || !sb->s_root || !(sb->s_flags & SB_ACTIVE)) {
> super_unlock_read(sb);
> return false;
> }
> @@ -1572,7 +1674,7 @@ int vfs_get_tree(struct fs_context *fc)
> * flag.
> */
> smp_wmb();
Is the barrier still needed here when super_wake() has smp_store_release()?
> - sb->s_flags |= SB_BORN;
> + super_wake(sb, SB_BORN);
I'm also kind of wondering whether when we have SB_BORN and SB_DYING isn't
the SB_ACTIVE flag redundant. SB_BORN is set practically at the same moment
as SB_ACTIVE. SB_ACTIVE gets cleared somewhat earlier than SB_DYING is set
but I believe SB_DYING can be set earlier (after all by the time SB_ACTIVE
is cleared we have sb->s_root == NULL which basically stops most of the
places looking at superblocks. As I'm grepping we've grown a lot of
SB_ACTIVE handling all over the place so this would be a bit non-trivial
but I belive it will make it easier for filesystem developers to decide
which flag they should be using... Also we could then drop sb->s_root
checks from many places because the locking helpers will return false if
SB_DYING is set.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
