Unverified Commit f0cd9880 authored by Christian Brauner's avatar Christian Brauner
Browse files

fs: massage locking helpers 

Multiple people have balked at the the fact that
super_lock{_shared,_excluse}() return booleans and even if they return
false hold s_umount. So let's change them to only hold s_umount when
true is returned and change the code accordingly.

Link: https://lore.kernel.org/r/20231024-vfs-super-freeze-v2-1-599c19f4faac@kernel.org


Reviewed-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent b85ea95d

fs/super.c

+61 −44
Original line number Diff line number Diff line
@@ -105,8 +105,9 @@ static inline bool wait_born(struct super_block *sb) 
 *

 * 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.

 * Return: The function returns true if SB_BORN was set and with

 *         s_umount held. The function returns false if SB_DYING was

 *         set and without s_umount held.

 */

static __must_check bool super_lock(struct super_block *sb, bool excl)

{
@@ -121,8 +122,10 @@ static __must_check bool super_lock(struct super_block *sb, bool excl) 
	 * @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)

	if (sb->s_flags & SB_DYING) {

		super_unlock(sb, excl);

		return false;

	}



	/* Has called ->get_tree() successfully. */

	if (sb->s_flags & SB_BORN)
@@ -140,13 +143,13 @@ static __must_check bool super_lock(struct super_block *sb, bool excl) 
	goto relock;

}



/* wait and acquire read-side of @sb->s_umount */

/* wait and try to 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 */

/* wait and try to acquire write-side of @sb->s_umount */

static inline bool super_lock_excl(struct super_block *sb)

{

	return super_lock(sb, true);
@@ -535,16 +538,18 @@ EXPORT_SYMBOL(deactivate_super); 
 */

static int grab_super(struct super_block *s) __releases(sb_lock)

{

	bool born;

	bool locked;



	s->s_count++;

	spin_unlock(&sb_lock);

	born = super_lock_excl(s);

	if (born && atomic_inc_not_zero(&s->s_active)) {

	locked = super_lock_excl(s);

	if (locked) {

		if (atomic_inc_not_zero(&s->s_active)) {

			put_super(s);

			return 1;

		}

		super_unlock_excl(s);

	}

	put_super(s);

	return 0;

}
@@ -575,7 +580,6 @@ static inline bool wait_dead(struct super_block *sb) 
 */

static bool grab_super_dead(struct super_block *sb)

{



	sb->s_count++;

	if (grab_super(sb)) {

		put_super(sb);
@@ -961,15 +965,17 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg) 


	spin_lock(&sb_lock);

	list_for_each_entry(sb, &super_blocks, s_list) {

		bool born;

		bool locked;



		sb->s_count++;

		spin_unlock(&sb_lock);



		born = super_lock_shared(sb);

		if (born && sb->s_root)

		locked = super_lock_shared(sb);

		if (locked) {

			if (sb->s_root)

				f(sb, arg);

			super_unlock_shared(sb);

		}



		spin_lock(&sb_lock);

		if (p)
@@ -997,15 +1003,17 @@ void iterate_supers_type(struct file_system_type *type, 


	spin_lock(&sb_lock);

	hlist_for_each_entry(sb, &type->fs_supers, s_instances) {

		bool born;

		bool locked;



		sb->s_count++;

		spin_unlock(&sb_lock);



		born = super_lock_shared(sb);

		if (born && sb->s_root)

		locked = super_lock_shared(sb);

		if (locked) {

			if (sb->s_root)

				f(sb, arg);

			super_unlock_shared(sb);

		}



		spin_lock(&sb_lock);

		if (p)
@@ -1054,15 +1062,17 @@ struct super_block *user_get_super(dev_t dev, bool excl) 
	spin_lock(&sb_lock);

	list_for_each_entry(sb, &super_blocks, s_list) {

		if (sb->s_dev ==  dev) {

			bool born;

			bool locked;



			sb->s_count++;

			spin_unlock(&sb_lock);

			/* still alive? */

			born = super_lock(sb, excl);

			if (born && sb->s_root)

			locked = super_lock(sb, excl);

			if (locked) {

				if (sb->s_root)

					return sb;

				super_unlock(sb, excl);

			}

			/* nope, got unmounted */

			spin_lock(&sb_lock);

			__put_super(sb);
@@ -1173,9 +1183,9 @@ int reconfigure_super(struct fs_context *fc) 


static void do_emergency_remount_callback(struct super_block *sb)

{

	bool born = super_lock_excl(sb);

	bool locked = super_lock_excl(sb);



	if (born && sb->s_root && sb->s_bdev && !sb_rdonly(sb)) {

	if (locked && sb->s_root && sb->s_bdev && !sb_rdonly(sb)) {

		struct fs_context *fc;



		fc = fs_context_for_reconfigure(sb->s_root,
@@ -1186,6 +1196,7 @@ static void do_emergency_remount_callback(struct super_block *sb) 
			put_fs_context(fc);

		}

	}

	if (locked)

		super_unlock_excl(sb);

}
@@ -1209,16 +1220,17 @@ void emergency_remount(void) 


static void do_thaw_all_callback(struct super_block *sb)

{

	bool born = super_lock_excl(sb);

	bool locked = super_lock_excl(sb);



	if (born && sb->s_root) {

	if (locked && sb->s_root) {

		if (IS_ENABLED(CONFIG_BLOCK))

			while (sb->s_bdev && !thaw_bdev(sb->s_bdev))

				pr_warn("Emergency Thaw on %pg\n", sb->s_bdev);

		thaw_super_locked(sb, FREEZE_HOLDER_USERSPACE);

	} else {

		super_unlock_excl(sb);

		return;

	}

	if (locked)

		super_unlock_excl(sb);

}



static void do_thaw_all(struct work_struct *work)
@@ -1432,7 +1444,7 @@ static struct super_block *bdev_super_lock_shared(struct block_device *bdev) 
	__releases(&bdev->bd_holder_lock)

{

	struct super_block *sb = bdev->bd_holder;

	bool born;

	bool locked;



	lockdep_assert_held(&bdev->bd_holder_lock);

	lockdep_assert_not_held(&sb->s_umount);
@@ -1444,9 +1456,8 @@ static struct super_block *bdev_super_lock_shared(struct block_device *bdev) 
	spin_unlock(&sb_lock);

	mutex_unlock(&bdev->bd_holder_lock);



	born = super_lock_shared(sb);

	if (!born || !sb->s_root || !(sb->s_flags & SB_ACTIVE)) {

		super_unlock_shared(sb);

	locked = super_lock_shared(sb);

	if (!locked || !sb->s_root || !(sb->s_flags & SB_ACTIVE)) {

		put_super(sb);

		return NULL;

	}
@@ -1962,9 +1973,11 @@ int freeze_super(struct super_block *sb, enum freeze_holder who) 
{

	int ret;



	if (!super_lock_excl(sb)) {

		WARN_ON_ONCE("Dying superblock while freezing!");

		return -EINVAL;

	}

	atomic_inc(&sb->s_active);

	if (!super_lock_excl(sb))

		WARN(1, "Dying superblock while freezing!");



retry:

	if (sb->s_writers.frozen == SB_FREEZE_COMPLETE) {
@@ -2012,8 +2025,10 @@ int freeze_super(struct super_block *sb, enum freeze_holder who) 
	/* Release s_umount to preserve sb_start_write -> s_umount ordering */

	super_unlock_excl(sb);

	sb_wait_write(sb, SB_FREEZE_WRITE);

	if (!super_lock_excl(sb))

		WARN(1, "Dying superblock while freezing!");

	if (!super_lock_excl(sb)) {

		WARN_ON_ONCE("Dying superblock while freezing!");

		return -EINVAL;

	}



	/* Now we go and block page faults... */

	sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
@@ -2131,8 +2146,10 @@ static int thaw_super_locked(struct super_block *sb, enum freeze_holder who) 
 */

int thaw_super(struct super_block *sb, enum freeze_holder who)

{

	if (!super_lock_excl(sb))

		WARN(1, "Dying superblock while thawing!");

	if (!super_lock_excl(sb)) {

		WARN_ON_ONCE("Dying superblock while thawing!");

		return -EINVAL;

	}

	return thaw_super_locked(sb, who);

}

EXPORT_SYMBOL(thaw_super);