parallel lookups: actual switch to rwsem

ta-da!

The main issue is the lack of down_write_killable(), so the places
like readdir.c switched to plain inode_lock(); once killable
variants of rwsem primitives appear, that'll be dealt with.

lockdep side also might need more work

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

11 files changed, 73 insertions, 32 deletions

diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 8810e2367fe6..1567a53857bd 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -539,3 +539,21 @@ in your dentry operations instead.
         it's a symlink.  Checking ->i_mode is really needed now.  In-tree we had
         to fix shmem_destroy_callback() that used to take that kind of shortcut;
         watch out, since that shortcut is no longer valid.
+--
+[mandatory]
+        ->i_mutex is replaced with ->i_rwsem now.  inode_lock() et.al. work as
+        they used to - they just take it exclusive.  However, ->lookup() may be
+        called with parent locked shared.  Its instances must not
+                * use d_instantiate) and d_rehash() separately - use d_add() or
+                  d_splice_alias() instead.
+                * use d_rehash() alone - call d_add(new_dentry, NULL) instead.
+                * in the unlikely case when (read-only) access to filesystem
+                  data structures needs exclusion for some reason, arrange it
+                  yourself.  None of the in-tree filesystems needed that.
+                * rely on ->d_parent and ->d_name not changing after dentry has
+                  been fed to d_add() or d_splice_alias().  Again, none of the
+                  in-tree instances relied upon that.
+        We are guaranteed that lookups of the same name in the same directory
+        will not happen in parallel ("same" in the sense of your ->d_compare()).
+        Lookups on different names in the same directory can and do happen in
+        parallel now.
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 5a23806ae418..0b8ba717175b 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -837,9 +837,11 @@ static noinline int btrfs_mksubvol(struct path *parent,
         struct dentry *dentry;
         int error;
 
-        error = mutex_lock_killable_nested(&dir->i_mutex, I_MUTEX_PARENT);
-        if (error == -EINTR)
-                return error;
+        inode_lock_nested(dir, I_MUTEX_PARENT);
+        // XXX: should've been
+        // mutex_lock_killable_nested(&dir->i_mutex, I_MUTEX_PARENT);
+        // if (error == -EINTR)
+        //      return error;
 
         dentry = lookup_one_len(name, parent->dentry, namelen);
         error = PTR_ERR(dentry);
@@ -2366,9 +2368,11 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
                 goto out;
 
 
-        err = mutex_lock_killable_nested(&dir->i_mutex, I_MUTEX_PARENT);
-        if (err == -EINTR)
-                goto out_drop_write;
+        inode_lock_nested(dir, I_MUTEX_PARENT);
+        // XXX: should've been
+        // err = mutex_lock_killable_nested(&dir->i_mutex, I_MUTEX_PARENT);
+        // if (err == -EINTR)
+        //      goto out_drop_write;
         dentry = lookup_one_len(vol_args->name, parent, namelen);
         if (IS_ERR(dentry)) {
                 err = PTR_ERR(dentry);
@@ -2558,7 +2562,7 @@ out_dput:
         dput(dentry);
 out_unlock_dir:
         inode_unlock(dir);
-out_drop_write:
+//out_drop_write:
         mnt_drop_write_file(file);
 out:
         kfree(vol_args);
diff --git a/fs/configfs/inode.c b/fs/configfs/inode.c
index 03d124ae27d7..0387968e6f47 100644
--- a/fs/configfs/inode.c
+++ b/fs/configfs/inode.c
@@ -156,7 +156,7 @@ static void configfs_set_inode_lock_class(struct configfs_dirent *sd,
 
         if (depth > 0) {
                 if (depth <= ARRAY_SIZE(default_group_class)) {
-                        lockdep_set_class(&inode->i_mutex,
+                        lockdep_set_class(&inode->i_rwsem,
                                           &default_group_class[depth - 1]);
                 } else {
                         /*
diff --git a/fs/dcache.c b/fs/dcache.c
index 59fcffcbf096..e49ba7d1b957 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2932,7 +2932,8 @@ struct dentry *d_ancestor(struct dentry *p1, struct dentry *p2)
 static int __d_unalias(struct inode *inode,
                 struct dentry *dentry, struct dentry *alias)
 {
-        struct mutex *m1 = NULL, *m2 = NULL;
+        struct mutex *m1 = NULL;
+        struct rw_semaphore *m2 = NULL;
         int ret = -ESTALE;
 
         /* If alias and dentry share a parent, then no extra locks required */
@@ -2943,15 +2944,15 @@ static int __d_unalias(struct inode *inode,
         if (!mutex_trylock(&dentry->d_sb->s_vfs_rename_mutex))
                 goto out_err;
         m1 = &dentry->d_sb->s_vfs_rename_mutex;
-        if (!inode_trylock(alias->d_parent->d_inode))
+        if (!inode_trylock_shared(alias->d_parent->d_inode))
                 goto out_err;
-        m2 = &alias->d_parent->d_inode->i_mutex;
+        m2 = &alias->d_parent->d_inode->i_rwsem;
 out_unalias:
         __d_move(alias, dentry, false);
         ret = 0;
 out_err:
         if (m2)
-                mutex_unlock(m2);
+                up_read(m2);
         if (m1)
                 mutex_unlock(m1);
         return ret;
diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c
index c09c63dcd7a2..45463600fb81 100644
--- a/fs/gfs2/ops_fstype.c
+++ b/fs/gfs2/ops_fstype.c
@@ -824,7 +824,7 @@ static int init_inodes(struct gfs2_sbd *sdp, int undo)
          * i_mutex on quota files is special. Since this inode is hidden system
          * file, we are safe to define locking ourselves.
          */
-        lockdep_set_class(&sdp->sd_quota_inode->i_mutex,
+        lockdep_set_class(&sdp->sd_quota_inode->i_rwsem,
                           &gfs2_quota_imutex_key);
 
         error = gfs2_rindex_update(sdp);
diff --git a/fs/inode.c b/fs/inode.c
index 4b884f73214e..4ccbc21b30ce 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -166,8 +166,8 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
         spin_lock_init(&inode->i_lock);
         lockdep_set_class(&inode->i_lock, &sb->s_type->i_lock_key);
 
-        mutex_init(&inode->i_mutex);
-        lockdep_set_class(&inode->i_mutex, &sb->s_type->i_mutex_key);
+        init_rwsem(&inode->i_rwsem);
+        lockdep_set_class(&inode->i_rwsem, &sb->s_type->i_mutex_key);
 
         atomic_set(&inode->i_dio_count, 0);
 
@@ -925,13 +925,13 @@ void lockdep_annotate_inode_mutex_key(struct inode *inode)
                 struct file_system_type *type = inode->i_sb->s_type;
 
                 /* Set new key only if filesystem hasn't already changed it */
-                if (lockdep_match_class(&inode->i_mutex, &type->i_mutex_key)) {
+                if (lockdep_match_class(&inode->i_rwsem, &type->i_mutex_key)) {
                         /*
                          * ensure nobody is actually holding i_mutex
                          */
-                        mutex_destroy(&inode->i_mutex);
-                        mutex_init(&inode->i_mutex);
-                        lockdep_set_class(&inode->i_mutex,
+                        // mutex_destroy(&inode->i_mutex);
+                        init_rwsem(&inode->i_rwsem);
+                        lockdep_set_class(&inode->i_rwsem,
                                           &type->i_mutex_dir_key);
                 }
         }
diff --git a/fs/namei.c b/fs/namei.c
index 7babb5e5f276..8249852b5fc6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1607,7 +1607,7 @@ static struct dentry *lookup_slow(const struct qstr *name,
         struct inode *inode = dir->d_inode;
         DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
 
-        inode_lock(inode);
+        inode_lock_shared(inode);
         /* Don't go there if it's already dead */
         if (unlikely(IS_DEADDIR(inode)))
                 goto out;
@@ -1638,7 +1638,7 @@ again:
                 }
         }
 out:
-        inode_unlock(inode);
+        inode_unlock_shared(inode);
         return dentry;
 }
 
diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
index 12f4a9e9800f..0748777f2e2a 100644
--- a/fs/ocfs2/inode.c
+++ b/fs/ocfs2/inode.c
@@ -262,7 +262,7 @@ static int ocfs2_init_locked_inode(struct inode *inode, void *opaque)
         inode->i_ino = args->fi_ino;
         OCFS2_I(inode)->ip_blkno = args->fi_blkno;
         if (args->fi_sysfile_type != 0)
-                lockdep_set_class(&inode->i_mutex,
+                lockdep_set_class(&inode->i_rwsem,
                         &ocfs2_sysfile_lock_key[args->fi_sysfile_type]);
         if (args->fi_sysfile_type == USER_QUOTA_SYSTEM_INODE ||
             args->fi_sysfile_type == GROUP_QUOTA_SYSTEM_INODE ||
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 6ec1e43a9a54..da186ee4f846 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -218,7 +218,9 @@ static int ovl_check_whiteouts(struct dentry *dir, struct ovl_readdir_data *rdd)
         cap_raise(override_cred->cap_effective, CAP_DAC_OVERRIDE);
         old_cred = override_creds(override_cred);
 
-        err = mutex_lock_killable(&dir->d_inode->i_mutex);
+        inode_lock(dir->d_inode);
+        err = 0;
+        // XXX: err = mutex_lock_killable(&dir->d_inode->i_mutex);
         if (!err) {
                 while (rdd->first_maybe_whiteout) {
                         p = rdd->first_maybe_whiteout;
diff --git a/fs/readdir.c b/fs/readdir.c
index e69ef3b79787..bf583e848a1a 100644
--- a/fs/readdir.c
+++ b/fs/readdir.c
@@ -32,9 +32,10 @@ int iterate_dir(struct file *file, struct dir_context *ctx)
         if (res)
                 goto out;
 
-        res = mutex_lock_killable(&inode->i_mutex);
-        if (res)
-                goto out;
+        inode_lock(inode);
+        // res = mutex_lock_killable(&inode->i_mutex);
+        // if (res)
+        //      goto out;
 
         res = -ENOENT;
         if (!IS_DEADDIR(inode)) {
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 00cecc5a2f75..3018f31f7aa0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -647,7 +647,7 @@ struct inode {
 
         /* Misc */
         unsigned long           i_state;
-        struct mutex            i_mutex;
+        struct rw_semaphore     i_rwsem;
 
         unsigned long           dirtied_when;   /* jiffies of first dirtying */
         unsigned long           dirtied_time_when;
@@ -734,27 +734,42 @@ enum inode_i_mutex_lock_class
 
 static inline void inode_lock(struct inode *inode)
 {
-        mutex_lock(&inode->i_mutex);
+        down_write(&inode->i_rwsem);
 }
 
 static inline void inode_unlock(struct inode *inode)
 {
-        mutex_unlock(&inode->i_mutex);
+        up_write(&inode->i_rwsem);
+}
+
+static inline void inode_lock_shared(struct inode *inode)
+{
+        down_read(&inode->i_rwsem);
+}
+
+static inline void inode_unlock_shared(struct inode *inode)
+{
+        up_read(&inode->i_rwsem);
 }
 
 static inline int inode_trylock(struct inode *inode)
 {
-        return mutex_trylock(&inode->i_mutex);
+        return down_write_trylock(&inode->i_rwsem);
+}
+
+static inline int inode_trylock_shared(struct inode *inode)
+{
+        return down_read_trylock(&inode->i_rwsem);
 }
 
 static inline int inode_is_locked(struct inode *inode)
 {
-        return mutex_is_locked(&inode->i_mutex);
+        return rwsem_is_locked(&inode->i_rwsem);
 }
 
 static inline void inode_lock_nested(struct inode *inode, unsigned subclass)
 {
-        mutex_lock_nested(&inode->i_mutex, subclass);
+        down_write_nested(&inode->i_rwsem, subclass);
 }
 
 void lock_two_nondirectories(struct inode *, struct inode*);