From 2c18a63b760a0f68f14cb8bb4c3840bb0b63b73e Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Fri, 18 Aug 2023 16:00:51 +0200 Subject: super: wait until we passed kill super Recent rework moved block device closing out of sb->put_super() and into sb->kill_sb() to avoid deadlocks as s_umount is held in put_super() and blkdev_put() can end up taking s_umount again. That means we need to move the removal of the superblock from @fs_supers out of generic_shutdown_super() and into deactivate_locked_super() to ensure that concurrent mounters don't fail to open block devices that are still in use because blkdev_put() in sb->kill_sb() hasn't been called yet. We can now do this as we can make iterators through @fs_super and @super_blocks wait without holding s_umount. Concurrent mounts will wait until a dying superblock is fully dead so until sb->kill_sb() has been called and SB_DEAD been set. Concurrent iterators can already discard any SB_DYING superblock. Reviewed-by: Jan Kara Message-Id: <20230818-vfs-super-fixes-v3-v3-4-9f0b1876e46b@kernel.org> Signed-off-by: Christian Brauner --- fs/super.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 64 insertions(+), 7 deletions(-) (limited to 'fs/super.c') diff --git a/fs/super.c b/fs/super.c index e2630fe4928a..2f604a6494fb 100644 --- a/fs/super.c +++ b/fs/super.c @@ -153,7 +153,7 @@ static inline bool super_lock_excl(struct super_block *sb) } /* wake waiters */ -#define SUPER_WAKE_FLAGS (SB_BORN | SB_DYING) +#define SUPER_WAKE_FLAGS (SB_BORN | SB_DYING | SB_DEAD) static void super_wake(struct super_block *sb, unsigned int flag) { WARN_ON_ONCE((flag & ~SUPER_WAKE_FLAGS)); @@ -461,6 +461,25 @@ void deactivate_locked_super(struct super_block *s) list_lru_destroy(&s->s_dentry_lru); list_lru_destroy(&s->s_inode_lru); + /* + * Remove it from @fs_supers so it isn't found by new + * sget{_fc}() walkers anymore. Any concurrent mounter still + * managing to grab a temporary reference is guaranteed to + * already see SB_DYING and will wait until we notify them about + * SB_DEAD. + */ + spin_lock(&sb_lock); + hlist_del_init(&s->s_instances); + spin_unlock(&sb_lock); + + /* + * Let concurrent mounts know that this thing is really dead. + * We don't need @sb->s_umount here as every concurrent caller + * will see SB_DYING and either discard the superblock or wait + * for SB_DEAD. + */ + super_wake(s, SB_DEAD); + put_filesystem(fs); put_super(s); } else { @@ -517,6 +536,45 @@ static int grab_super(struct super_block *s) __releases(sb_lock) return 0; } +static inline bool wait_dead(struct super_block *sb) +{ + unsigned int flags; + + /* + * Pairs with memory barrier in super_wake() and ensures + * that we see SB_DEAD after we're woken. + */ + flags = smp_load_acquire(&sb->s_flags); + return flags & SB_DEAD; +} + +/** + * grab_super_dead - acquire an active reference to a superblock + * @sb: superblock to acquire + * + * Acquire a temporary reference on a superblock and try to trade it for + * an active reference. This is used in sget{_fc}() to wait for a + * superblock to either become SB_BORN or for it to pass through + * sb->kill() and be marked as SB_DEAD. + * + * Return: This returns true if an active reference could be acquired, + * false if not. + */ +static bool grab_super_dead(struct super_block *sb) +{ + + sb->s_count++; + if (grab_super(sb)) { + put_super(sb); + lockdep_assert_held(&sb->s_umount); + return true; + } + wait_var_event(&sb->s_flags, wait_dead(sb)); + put_super(sb); + lockdep_assert_not_held(&sb->s_umount); + return false; +} + /* * super_trylock_shared - try to grab ->s_umount shared * @sb: reference we are trying to grab @@ -643,15 +701,14 @@ void generic_shutdown_super(struct super_block *sb) spin_unlock(&sb->s_inode_list_lock); } } - spin_lock(&sb_lock); - /* 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. + * + * We leave the superblock on @fs_supers so it can be found by + * sget{_fc}() until we passed sb->kill_sb(). */ super_wake(sb, SB_DYING); super_unlock_excl(sb); @@ -746,7 +803,7 @@ share_extant_sb: destroy_unused_super(s); return ERR_PTR(-EBUSY); } - if (!grab_super(old)) + if (!grab_super_dead(old)) goto retry; destroy_unused_super(s); return old; @@ -790,7 +847,7 @@ retry: destroy_unused_super(s); return ERR_PTR(-EBUSY); } - if (!grab_super(old)) + if (!grab_super_dead(old)) goto retry; destroy_unused_super(s); return old; -- cgit