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Date: Thu, 21 May 2020 19:18:49 +0300
From: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To: David Howells <dhowells@...hat.com>, Theodore Ts'o <tytso@....edu>,
Andreas Dilger <adilger.kernel@...ger.ca>,
Alexander Viro <viro@...iv.linux.org.uk>
Cc: linux-afs@...ts.infradead.org, linux-ext4@...r.kernel.org,
linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs, afs, ext4: Make the inode hash table RCU searchable
On 21/05/2020 18.39, David Howells wrote:
> Hi Ted, Andreas, Konstantin,
>
> Is this something that would be of interest to Ext4?
For now, I've plugged this issue with try-lock in ext4 lazy time update.
This solution is much better.
I guess next step is rcu-protected fast-path in iget_locked()?
(for scalable open_by_handle_at and nfs exports)
>
> David
> ---
> vfs, afs, ext4: Make the inode hash table RCU searchable
>
> Make the inode hash table RCU searchable so that searches that want to
> access or modify an inode without taking a ref on that inode can do so
> without taking the inode hash table lock.
>
> The main thing this requires is some RCU annotation on the list
> manipulation operations. Inodes are already freed by RCU in most cases.
>
> Users of this interface must take care as the inode may be still under
> construction or may be being torn down around them.
>
> There are at least two instances where this can be of use:
>
> (1) Ext4 date stamp updating.
>
> (2) AFS callback breaking.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> cc: linux-ext4@...r.kernel.org
> ---
> fs/afs/callback.c | 12 ++-
> fs/ext4/inode.c | 44 ++++++-------
> fs/inode.c | 173 ++++++++++++++++++++++++++++++++++++++++++++---------
> include/linux/fs.h | 3
> 4 files changed, 179 insertions(+), 53 deletions(-)
>
> diff --git a/fs/afs/callback.c b/fs/afs/callback.c
> index 2dca8df1a18d..0dcbd40732d1 100644
> --- a/fs/afs/callback.c
> +++ b/fs/afs/callback.c
> @@ -252,6 +252,7 @@ static void afs_break_one_callback(struct afs_server *server,
> struct afs_vnode *vnode;
> struct inode *inode;
>
> + rcu_read_lock();
> read_lock(&server->cb_break_lock);
> hlist_for_each_entry(vi, &server->cb_volumes, srv_link) {
> if (vi->vid < fid->vid)
> @@ -287,12 +288,16 @@ static void afs_break_one_callback(struct afs_server *server,
> } else {
> data.volume = NULL;
> data.fid = *fid;
> - inode = ilookup5_nowait(cbi->sb, fid->vnode,
> - afs_iget5_test, &data);
> +
> + /* See if we can find a matching inode - even an I_NEW
> + * inode needs to be marked as it can have its callback
> + * broken before we finish setting up the local inode.
> + */
> + inode = find_inode_rcu(cbi->sb, fid->vnode,
> + afs_iget5_test, &data);
> if (inode) {
> vnode = AFS_FS_I(inode);
> afs_break_callback(vnode, afs_cb_break_for_callback);
> - iput(inode);
> } else {
> trace_afs_cb_miss(fid, afs_cb_break_for_callback);
> }
> @@ -301,6 +306,7 @@ static void afs_break_one_callback(struct afs_server *server,
>
> out:
> read_unlock(&server->cb_break_lock);
> + rcu_read_unlock();
> }
>
> /*
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 2a4aae6acdcb..2bbb55d05bb7 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4860,21 +4860,22 @@ static int ext4_inode_blocks_set(handle_t *handle,
> return 0;
> }
>
> -struct other_inode {
> - unsigned long orig_ino;
> - struct ext4_inode *raw_inode;
> -};
> -
> -static int other_inode_match(struct inode * inode, unsigned long ino,
> - void *data)
> +static void __ext4_update_other_inode_time(struct super_block *sb,
> + unsigned long orig_ino,
> + unsigned long ino,
> + struct ext4_inode *raw_inode)
> {
> - struct other_inode *oi = (struct other_inode *) data;
> + struct inode *inode;
> +
> + inode = find_inode_by_ino_rcu(sb, ino);
> + if (!inode)
> + return;
>
> - if ((inode->i_ino != ino) ||
> - (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW |
> + if ((inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW |
> I_DIRTY_INODE)) ||
> ((inode->i_state & I_DIRTY_TIME) == 0))
> - return 0;
> + return;
> +
> spin_lock(&inode->i_lock);
> if (((inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW |
> I_DIRTY_INODE)) == 0) &&
> @@ -4885,16 +4886,15 @@ static int other_inode_match(struct inode * inode, unsigned long ino,
> spin_unlock(&inode->i_lock);
>
> spin_lock(&ei->i_raw_lock);
> - EXT4_INODE_SET_XTIME(i_ctime, inode, oi->raw_inode);
> - EXT4_INODE_SET_XTIME(i_mtime, inode, oi->raw_inode);
> - EXT4_INODE_SET_XTIME(i_atime, inode, oi->raw_inode);
> - ext4_inode_csum_set(inode, oi->raw_inode, ei);
> + EXT4_INODE_SET_XTIME(i_ctime, inode, raw_inode);
> + EXT4_INODE_SET_XTIME(i_mtime, inode, raw_inode);
> + EXT4_INODE_SET_XTIME(i_atime, inode, raw_inode);
> + ext4_inode_csum_set(inode, raw_inode, ei);
> spin_unlock(&ei->i_raw_lock);
> - trace_ext4_other_inode_update_time(inode, oi->orig_ino);
> - return -1;
> + trace_ext4_other_inode_update_time(inode, orig_ino);
> + return;
> }
> spin_unlock(&inode->i_lock);
> - return -1;
> }
>
> /*
> @@ -4904,24 +4904,24 @@ static int other_inode_match(struct inode * inode, unsigned long ino,
> static void ext4_update_other_inodes_time(struct super_block *sb,
> unsigned long orig_ino, char *buf)
> {
> - struct other_inode oi;
> unsigned long ino;
> int i, inodes_per_block = EXT4_SB(sb)->s_inodes_per_block;
> int inode_size = EXT4_INODE_SIZE(sb);
>
> - oi.orig_ino = orig_ino;
> /*
> * Calculate the first inode in the inode table block. Inode
> * numbers are one-based. That is, the first inode in a block
> * (assuming 4k blocks and 256 byte inodes) is (n*16 + 1).
> */
> ino = ((orig_ino - 1) & ~(inodes_per_block - 1)) + 1;
> + rcu_read_lock();
> for (i = 0; i < inodes_per_block; i++, ino++, buf += inode_size) {
> if (ino == orig_ino)
> continue;
> - oi.raw_inode = (struct ext4_inode *) buf;
> - (void) find_inode_nowait(sb, ino, other_inode_match, &oi);
> + __ext4_update_other_inode_time(sb, orig_ino, ino,
> + (struct ext4_inode *)buf);
> }
> + rcu_read_unlock();
> }
>
> /*
> diff --git a/fs/inode.c b/fs/inode.c
> index 93d9252a00ab..a9ae3a405a1f 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -497,7 +497,7 @@ void __insert_inode_hash(struct inode *inode, unsigned long hashval)
>
> spin_lock(&inode_hash_lock);
> spin_lock(&inode->i_lock);
> - hlist_add_head(&inode->i_hash, b);
> + hlist_add_head_rcu(&inode->i_hash, b);
> spin_unlock(&inode->i_lock);
> spin_unlock(&inode_hash_lock);
> }
> @@ -513,7 +513,7 @@ void __remove_inode_hash(struct inode *inode)
> {
> spin_lock(&inode_hash_lock);
> spin_lock(&inode->i_lock);
> - hlist_del_init(&inode->i_hash);
> + hlist_del_init_rcu(&inode->i_hash);
> spin_unlock(&inode->i_lock);
> spin_unlock(&inode_hash_lock);
> }
> @@ -808,8 +808,31 @@ long prune_icache_sb(struct super_block *sb, struct shrink_control *sc)
> }
>
> static void __wait_on_freeing_inode(struct inode *inode);
> +
> /*
> - * Called with the inode lock held.
> + * Find an inode. Can be called with either the RCU read lock or the
> + * inode cache lock held. No check is made as to the validity of the
> + * inode found.
> + */
> +static struct inode *__find_inode_rcu(struct super_block *sb,
> + struct hlist_head *head,
> + int (*test)(struct inode *, void *),
> + void *data)
> +{
> + struct inode *inode;
> +
> + hlist_for_each_entry_rcu(inode, head, i_hash) {
> + if (inode->i_sb == sb &&
> + test(inode, data))
> + return inode;
> + }
> +
> + return NULL;
> +}
> +
> +/*
> + * Called with the inode hash lock held. Waits until dying inodes are freed,
> + * dropping the inode hash lock temporarily to do so.
> */
> static struct inode *find_inode(struct super_block *sb,
> struct hlist_head *head,
> @@ -819,11 +842,8 @@ static struct inode *find_inode(struct super_block *sb,
> struct inode *inode = NULL;
>
> repeat:
> - hlist_for_each_entry(inode, head, i_hash) {
> - if (inode->i_sb != sb)
> - continue;
> - if (!test(inode, data))
> - continue;
> + inode = __find_inode_rcu(sb, head, test, data);
> + if (inode) {
> spin_lock(&inode->i_lock);
> if (inode->i_state & (I_FREEING|I_WILL_FREE)) {
> __wait_on_freeing_inode(inode);
> @@ -840,6 +860,26 @@ static struct inode *find_inode(struct super_block *sb,
> return NULL;
> }
>
> +/*
> + * Find an inode by inode number. Can be called with either the RCU
> + * read lock or the inode cache lock held. No check is made as to the
> + * validity of the inode found.
> + */
> +static struct inode *__find_inode_by_ino_rcu(struct super_block *sb,
> + struct hlist_head *head,
> + unsigned long ino)
> +{
> + struct inode *inode;
> +
> + hlist_for_each_entry_rcu(inode, head, i_hash) {
> + if (inode->i_ino == ino &&
> + inode->i_sb == sb)
> + return inode;
> + }
> +
> + return NULL;
> +}
> +
> /*
> * find_inode_fast is the fast path version of find_inode, see the comment at
> * iget_locked for details.
> @@ -850,11 +890,8 @@ static struct inode *find_inode_fast(struct super_block *sb,
> struct inode *inode = NULL;
>
> repeat:
> - hlist_for_each_entry(inode, head, i_hash) {
> - if (inode->i_ino != ino)
> - continue;
> - if (inode->i_sb != sb)
> - continue;
> + inode = __find_inode_by_ino_rcu(sb, head, ino);
> + if (inode) {
> spin_lock(&inode->i_lock);
> if (inode->i_state & (I_FREEING|I_WILL_FREE)) {
> __wait_on_freeing_inode(inode);
> @@ -1107,7 +1144,7 @@ struct inode *inode_insert5(struct inode *inode, unsigned long hashval,
> */
> spin_lock(&inode->i_lock);
> inode->i_state |= I_NEW;
> - hlist_add_head(&inode->i_hash, head);
> + hlist_add_head_rcu(&inode->i_hash, head);
> spin_unlock(&inode->i_lock);
> if (!creating)
> inode_sb_list_add(inode);
> @@ -1201,7 +1238,7 @@ struct inode *iget_locked(struct super_block *sb, unsigned long ino)
> inode->i_ino = ino;
> spin_lock(&inode->i_lock);
> inode->i_state = I_NEW;
> - hlist_add_head(&inode->i_hash, head);
> + hlist_add_head_rcu(&inode->i_hash, head);
> spin_unlock(&inode->i_lock);
> inode_sb_list_add(inode);
> spin_unlock(&inode_hash_lock);
> @@ -1245,15 +1282,9 @@ static int test_inode_iunique(struct super_block *sb, unsigned long ino)
> struct inode *inode;
>
> spin_lock(&inode_hash_lock);
> - hlist_for_each_entry(inode, b, i_hash) {
> - if (inode->i_ino == ino && inode->i_sb == sb) {
> - spin_unlock(&inode_hash_lock);
> - return 0;
> - }
> - }
> + inode = __find_inode_by_ino_rcu(sb, b, ino);
> spin_unlock(&inode_hash_lock);
> -
> - return 1;
> + return inode ? 0 : 1;
> }
>
> /**
> @@ -1325,6 +1356,7 @@ EXPORT_SYMBOL(igrab);
> *
> * Note: I_NEW is not waited upon so you have to be very careful what you do
> * with the returned inode. You probably should be using ilookup5() instead.
> + * It may still sleep waiting for I_FREE and I_WILL_FREE, however.
> *
> * Note2: @test is called with the inode_hash_lock held, so can't sleep.
> */
> @@ -1456,6 +1488,86 @@ struct inode *find_inode_nowait(struct super_block *sb,
> }
> EXPORT_SYMBOL(find_inode_nowait);
>
> +/**
> + * find_inode_rcu - find an inode in the inode cache
> + * @sb: Super block of file system to search
> + * @hashval: Key to hash
> + * @test: Function to test match on an inode
> + * @data: Data for test function
> + *
> + * Search for the inode specified by @hashval and @data in the inode cache,
> + * where the helper function @test will return 0 if the inode does not match
> + * and 1 if it does. The @test function must be responsible for taking the
> + * i_lock spin_lock and checking i_state for an inode being freed or being
> + * initialized.
> + *
> + * If successful, this will return the inode for which the @test function
> + * returned 1 and NULL otherwise.
> + *
> + * The @test function is not permitted to take a ref on any inode presented
> + * unless the caller is holding the inode hashtable lock. It is also not
> + * permitted to sleep, since it may be called with the RCU read lock held.
> + *
> + * The caller must hold either the RCU read lock or the inode hashtable lock.
> + */
> +struct inode *find_inode_rcu(struct super_block *sb, unsigned long hashval,
> + int (*test)(struct inode *, void *), void *data)
> +{
> + struct hlist_head *head = inode_hashtable + hash(sb, hashval);
> + struct inode *inode;
> +
> + RCU_LOCKDEP_WARN(!lockdep_is_held(&inode_hash_lock) && !rcu_read_lock_held(),
> + "suspicious find_inode_by_ino_rcu() usage");
> +
> + hlist_for_each_entry_rcu(inode, head, i_hash) {
> + if (inode->i_sb == sb &&
> + !(READ_ONCE(inode->i_state) & (I_FREEING | I_WILL_FREE)) &&
> + test(inode, data))
> + return inode;
> + }
> + return NULL;
> +}
> +EXPORT_SYMBOL(find_inode_rcu);
> +
> +/**
> + * find_inode_by_rcu - Find an inode in the inode cache
> + * @sb: Super block of file system to search
> + * @ino: The inode number to match
> + *
> + * Search for the inode specified by @hashval and @data in the inode cache,
> + * where the helper function @test will return 0 if the inode does not match
> + * and 1 if it does. The @test function must be responsible for taking the
> + * i_lock spin_lock and checking i_state for an inode being freed or being
> + * initialized.
> + *
> + * If successful, this will return the inode for which the @test function
> + * returned 1 and NULL otherwise.
> + *
> + * The @test function is not permitted to take a ref on any inode presented
> + * unless the caller is holding the inode hashtable lock. It is also not
> + * permitted to sleep, since it may be called with the RCU read lock held.
> + *
> + * The caller must hold either the RCU read lock or the inode hashtable lock.
> + */
> +struct inode *find_inode_by_ino_rcu(struct super_block *sb,
> + unsigned long ino)
> +{
> + struct hlist_head *head = inode_hashtable + hash(sb, ino);
> + struct inode *inode;
> +
> + RCU_LOCKDEP_WARN(!lockdep_is_held(&inode_hash_lock) && !rcu_read_lock_held(),
> + "suspicious find_inode_by_ino_rcu() usage");
> +
> + hlist_for_each_entry_rcu(inode, head, i_hash) {
> + if (inode->i_ino == ino &&
> + inode->i_sb == sb &&
> + !(READ_ONCE(inode->i_state) & (I_FREEING | I_WILL_FREE)))
> + return inode;
> + }
> + return NULL;
> +}
> +EXPORT_SYMBOL(find_inode_by_ino_rcu);
> +
> int insert_inode_locked(struct inode *inode)
> {
> struct super_block *sb = inode->i_sb;
> @@ -1480,7 +1592,7 @@ int insert_inode_locked(struct inode *inode)
> if (likely(!old)) {
> spin_lock(&inode->i_lock);
> inode->i_state |= I_NEW | I_CREATING;
> - hlist_add_head(&inode->i_hash, head);
> + hlist_add_head_rcu(&inode->i_hash, head);
> spin_unlock(&inode->i_lock);
> spin_unlock(&inode_hash_lock);
> return 0;
> @@ -1540,6 +1652,7 @@ static void iput_final(struct inode *inode)
> {
> struct super_block *sb = inode->i_sb;
> const struct super_operations *op = inode->i_sb->s_op;
> + unsigned long state;
> int drop;
>
> WARN_ON(inode->i_state & I_NEW);
> @@ -1555,16 +1668,20 @@ static void iput_final(struct inode *inode)
> return;
> }
>
> + state = READ_ONCE(inode->i_state);
> if (!drop) {
> - inode->i_state |= I_WILL_FREE;
> + WRITE_ONCE(inode->i_state, state | I_WILL_FREE);
> spin_unlock(&inode->i_lock);
> +
> write_inode_now(inode, 1);
> +
> spin_lock(&inode->i_lock);
> - WARN_ON(inode->i_state & I_NEW);
> - inode->i_state &= ~I_WILL_FREE;
> + state = READ_ONCE(inode->i_state);
> + WARN_ON(state & I_NEW);
> + state &= ~I_WILL_FREE;
> }
>
> - inode->i_state |= I_FREEING;
> + WRITE_ONCE(inode->i_state, state | I_FREEING);
> if (!list_empty(&inode->i_lru))
> inode_lru_list_del(inode);
> spin_unlock(&inode->i_lock);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 45cc10cdf6dd..5f9b2bb4b44f 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3070,6 +3070,9 @@ extern struct inode *find_inode_nowait(struct super_block *,
> int (*match)(struct inode *,
> unsigned long, void *),
> void *data);
> +extern struct inode *find_inode_rcu(struct super_block *, unsigned long,
> + int (*)(struct inode *, void *), void *);
> +extern struct inode *find_inode_by_ino_rcu(struct super_block *, unsigned long);
> extern int insert_inode_locked4(struct inode *, unsigned long, int (*test)(struct inode *, void *), void *);
> extern int insert_inode_locked(struct inode *);
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
>
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