Skip to content

Commit

Permalink
super: wait for nascent superblocks
Browse files Browse the repository at this point in the history 
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 aca740c ("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: aca740c ("fs: open block device after superblock creation")
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230818-vfs-super-fixes-v3-v3-3-9f0b1876e46b@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
  • Loading branch information
Christian Brauner committed Aug 21, 2023
1 parent d8ce82e commit 5e87491
Show file tree
Hide file tree
Showing 2 changed files with 154 additions and 51 deletions.
204 changes: 153 additions & 51 deletions fs/super.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -50,7 +50,7 @@ static char *sb_writers_name[SB_FREEZE_LEVELS] = {
"sb_internal",
};

static inline void super_lock(struct super_block *sb, bool excl)
static inline void __super_lock(struct super_block *sb, bool excl)
{
if (excl)
down_write(&sb->s_umount);
Expand All @@ -66,14 +66,9 @@ static inline void super_unlock(struct super_block *sb, bool excl)
up_read(&sb->s_umount);
}

static inline void super_lock_excl(struct super_block *sb)
static inline void __super_lock_excl(struct super_block *sb)
{
super_lock(sb, true);
}

static inline void super_lock_shared(struct super_block *sb)
{
super_lock(sb, false);
__super_lock(sb, true);
}

static inline void super_unlock_excl(struct super_block *sb)
Expand All @@ -86,6 +81,99 @@ 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 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 __must_check bool super_lock(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_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)
{
WARN_ON_ONCE((flag & ~SUPER_WAKE_FLAGS));
WARN_ON_ONCE(hweight32(flag & SUPER_WAKE_FLAGS) > 1);

/*
* Pairs with smp_load_acquire() in super_lock() to make sure
* all initializations in the superblock are seen by the user
* seeing SB_BORN sent.
*/
smp_store_release(&sb->s_flags, sb->s_flags | flag);
/*
* Pairs with the barrier in prepare_to_wait_event() to make sure
* ___wait_var_event() either sees SB_BORN set or
* waitqueue_active() check in wake_up_var() sees the waiter.
*/
smp_mb();
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.
Expand Down Expand Up @@ -393,7 +481,7 @@ EXPORT_SYMBOL(deactivate_locked_super);
void deactivate_super(struct super_block *s)
{
if (!atomic_add_unless(&s->s_active, -1, 1)) {
super_lock_excl(s);
__super_lock_excl(s);
deactivate_locked_super(s);
}
}
Expand All @@ -415,10 +503,12 @@ 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_excl(s);
if ((s->s_flags & SB_BORN) && atomic_inc_not_zero(&s->s_active)) {
born = super_lock_excl(s);
if (born && atomic_inc_not_zero(&s->s_active)) {
put_super(s);
return 1;
}
Expand Down Expand Up @@ -447,8 +537,8 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
bool super_trylock_shared(struct super_block *sb)
{
if (down_read_trylock(&sb->s_umount)) {
if (!hlist_unhashed(&sb->s_instances) &&
sb->s_root && (sb->s_flags & SB_BORN))
if (!(sb->s_flags & SB_DYING) && sb->s_root &&
(sb->s_flags & SB_BORN))
return true;
super_unlock_shared(sb);
}
Expand All @@ -475,7 +565,7 @@ bool super_trylock_shared(struct super_block *sb)
void retire_super(struct super_block *sb)
{
WARN_ON(!sb->s_bdev);
super_lock_excl(sb);
__super_lock_excl(sb);
if (sb->s_iflags & SB_I_PERSB_BDI) {
bdi_unregister(sb->s_bdi);
sb->s_iflags &= ~SB_I_PERSB_BDI;
Expand Down Expand Up @@ -557,6 +647,13 @@ 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_excl(sb);
if (sb->s_bdi != &noop_backing_dev_info) {
if (sb->s_iflags & SB_I_PERSB_BDI)
Expand Down Expand Up @@ -631,6 +728,11 @@ 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.
*/
list_add_tail(&s->s_list, &super_blocks);
hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
spin_unlock(&sb_lock);
Expand Down Expand Up @@ -740,7 +842,8 @@ static void __iterate_supers(void (*f)(struct super_block *))

spin_lock(&sb_lock);
list_for_each_entry(sb, &super_blocks, s_list) {
if (hlist_unhashed(&sb->s_instances))
/* Pairs with memory marrier in super_wake(). */
if (smp_load_acquire(&sb->s_flags) & SB_DYING)
continue;
sb->s_count++;
spin_unlock(&sb_lock);
Expand Down Expand Up @@ -770,13 +873,13 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)

spin_lock(&sb_lock);
list_for_each_entry(sb, &super_blocks, s_list) {
if (hlist_unhashed(&sb->s_instances))
continue;
bool born;

sb->s_count++;
spin_unlock(&sb_lock);

super_lock_shared(sb);
if (sb->s_root && (sb->s_flags & SB_BORN))
born = super_lock_shared(sb);
if (born && sb->s_root)
f(sb, arg);
super_unlock_shared(sb);

Expand Down Expand Up @@ -806,11 +909,13 @@ 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;

sb->s_count++;
spin_unlock(&sb_lock);

super_lock_shared(sb);
if (sb->s_root && (sb->s_flags & SB_BORN))
born = super_lock_shared(sb);
if (born && sb->s_root)
f(sb, arg);
super_unlock_shared(sb);

Expand Down Expand Up @@ -841,14 +946,11 @@ 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;
return NULL;
super_unlock_excl(sb);
return sb;
}
Expand All @@ -862,22 +964,21 @@ struct super_block *user_get_super(dev_t dev, bool excl)
struct super_block *sb;

spin_lock(&sb_lock);
rescan:
list_for_each_entry(sb, &super_blocks, s_list) {
if (hlist_unhashed(&sb->s_instances))
continue;
if (sb->s_dev == dev) {
bool born;

sb->s_count++;
spin_unlock(&sb_lock);
super_lock(sb, excl);
/* still alive? */
if (sb->s_root && (sb->s_flags & SB_BORN))
born = super_lock(sb, excl);
if (born && sb->s_root)
return sb;
super_unlock(sb, excl);
/* nope, got unmounted */
spin_lock(&sb_lock);
__put_super(sb);
goto rescan;
break;
}
}
spin_unlock(&sb_lock);
Expand Down Expand Up @@ -921,7 +1022,7 @@ int reconfigure_super(struct fs_context *fc)
if (!hlist_empty(&sb->s_pins)) {
super_unlock_excl(sb);
group_pin_kill(&sb->s_pins);
super_lock_excl(sb);
__super_lock_excl(sb);
if (!sb->s_root)
return 0;
if (sb->s_writers.frozen != SB_UNFROZEN)
Expand Down Expand Up @@ -984,9 +1085,9 @@ int reconfigure_super(struct fs_context *fc)

static void do_emergency_remount_callback(struct super_block *sb)
{
super_lock_excl(sb);
if (sb->s_root && sb->s_bdev && (sb->s_flags & SB_BORN) &&
!sb_rdonly(sb)) {
bool born = super_lock_excl(sb);

if (born && sb->s_root && sb->s_bdev && !sb_rdonly(sb)) {
struct fs_context *fc;

fc = fs_context_for_reconfigure(sb->s_root,
Expand Down Expand Up @@ -1020,8 +1121,9 @@ void emergency_remount(void)

static void do_thaw_all_callback(struct super_block *sb)
{
super_lock_excl(sb);
if (sb->s_root && sb->s_flags & SB_BORN) {
bool born = super_lock_excl(sb);

if (born && sb->s_root) {
emergency_thaw_bdev(sb);
thaw_super_locked(sb);
} else {
Expand Down Expand Up @@ -1212,9 +1314,9 @@ EXPORT_SYMBOL(get_tree_keyed);
*/
static bool super_lock_shared_active(struct super_block *sb)
{
super_lock_shared(sb);
if (!sb->s_root ||
(sb->s_flags & (SB_ACTIVE | SB_BORN)) != (SB_ACTIVE | SB_BORN)) {
bool born = super_lock_shared(sb);

if (!born || !sb->s_root || !(sb->s_flags & SB_ACTIVE)) {
super_unlock_shared(sb);
return false;
}
Expand Down Expand Up @@ -1374,7 +1476,7 @@ int get_tree_bdev(struct fs_context *fc,
*/
super_unlock_excl(s);
error = setup_bdev_super(s, fc->sb_flags, fc);
super_lock_excl(s);
__super_lock_excl(s);
if (!error)
error = fill_super(s, fc);
if (error) {
Expand Down Expand Up @@ -1426,7 +1528,7 @@ struct dentry *mount_bdev(struct file_system_type *fs_type,
*/
super_unlock_excl(s);
error = setup_bdev_super(s, flags, NULL);
super_lock_excl(s);
__super_lock_excl(s);
if (!error)
error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
if (error) {
Expand Down Expand Up @@ -1566,13 +1668,13 @@ int vfs_get_tree(struct fs_context *fc)
WARN_ON(!sb->s_bdi);

/*
* Write barrier is for super_cache_count(). We place it before setting
* SB_BORN as the data dependency between the two functions is the
* superblock structure contents that we just set up, not the SB_BORN
* flag.
* super_wake() contains a memory barrier which also care of
* ordering for super_cache_count(). We place it before setting
* SB_BORN as the data dependency between the two functions is
* the superblock structure contents that we just set up, not
* the SB_BORN flag.
*/
smp_wmb();
sb->s_flags |= SB_BORN;
super_wake(sb, SB_BORN);

error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
if (unlikely(error)) {
Expand Down Expand Up @@ -1715,7 +1817,7 @@ int freeze_super(struct super_block *sb)
int ret;

atomic_inc(&sb->s_active);
super_lock_excl(sb);
__super_lock_excl(sb);
if (sb->s_writers.frozen != SB_UNFROZEN) {
deactivate_locked_super(sb);
return -EBUSY;
Expand All @@ -1737,7 +1839,7 @@ int freeze_super(struct super_block *sb)
/* Release s_umount to preserve sb_start_write -> s_umount ordering */
super_unlock_excl(sb);
sb_wait_write(sb, SB_FREEZE_WRITE);
super_lock_excl(sb);
__super_lock_excl(sb);

/* Now we go and block page faults... */
sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
Expand Down Expand Up @@ -1820,7 +1922,7 @@ static int thaw_super_locked(struct super_block *sb)
*/
int thaw_super(struct super_block *sb)
{
super_lock_excl(sb);
__super_lock_excl(sb);
return thaw_super_locked(sb);
}
EXPORT_SYMBOL(thaw_super);
Expand Down
Loading

0 comments on commit 5e87491

Please sign in to comment.