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authorChristian Brauner <brauner@kernel.org>2023-08-18 16:00:51 +0200
committerChristian Brauner <brauner@kernel.org>2023-08-21 18:09:08 +0200
commit2c18a63b760a0f68f14cb8bb4c3840bb0b63b73e (patch)
tree08544495066d5e714d17fb3cb253cda95e37cbf7 /fs/super.c
parentsuper: wait for nascent superblocks (diff)
downloadlinux-2c18a63b760a0f68f14cb8bb4c3840bb0b63b73e.tar.xz
linux-2c18a63b760a0f68f14cb8bb4c3840bb0b63b73e.zip
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 <jack@suse.cz> Message-Id: <20230818-vfs-super-fixes-v3-v3-4-9f0b1876e46b@kernel.org> Signed-off-by: Christian Brauner <brauner@kernel.org>
Diffstat (limited to 'fs/super.c')
-rw-r--r--fs/super.c71
1 files changed, 64 insertions, 7 deletions
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;