[PATCH] fuse: simplify locking

This is in preparation for removing the global spinlock in favor of a
per-mount one.

The only critical part is the interaction between fuse_dev_release() and
fuse_fill_super(): fuse_dev_release() must see the assignment to
file->private_data, otherwise it will leak the reference to fuse_conn.

This is ensured by the fput() operation, which will synchronize the assignment
with other CPU's that may do a final fput() soon after this.

Also redundant locking is removed from fuse_fill_super(), where exclusion is
already ensured by the BKL held for this function by the VFS.

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 33 insertions, 62 deletions

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 75c6e9166c39..c510533c6849 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -23,13 +23,11 @@ static kmem_cache_t *fuse_req_cachep;
 
 static struct fuse_conn *fuse_get_conn(struct file *file)
 {
-        struct fuse_conn *fc;
-        spin_lock(&fuse_lock);
-        fc = file->private_data;
-        if (fc && !fc->connected)
-                fc = NULL;
-        spin_unlock(&fuse_lock);
-        return fc;
+        /*
+         * Lockless access is OK, because file->private data is set
+         * once during mount and is valid until the file is released.
+         */
+        return file->private_data;
 }
 
 static void fuse_request_init(struct fuse_req *req)
@@ -607,19 +605,16 @@ static ssize_t fuse_dev_readv(struct file *file, const struct iovec *iov,
                               unsigned long nr_segs, loff_t *off)
 {
         int err;
-        struct fuse_conn *fc;
         struct fuse_req *req;
         struct fuse_in *in;
         struct fuse_copy_state cs;
         unsigned reqsize;
+        struct fuse_conn *fc = fuse_get_conn(file);
+        if (!fc)
+                return -EPERM;
 
  restart:
         spin_lock(&fuse_lock);
-        fc = file->private_data;
-        err = -EPERM;
-        if (!fc)
-                goto err_unlock;
-
         err = -EAGAIN;
         if ((file->f_flags & O_NONBLOCK) && fc->connected &&
             list_empty(&fc->pending))
@@ -915,17 +910,13 @@ void fuse_abort_conn(struct fuse_conn *fc)
 
 static int fuse_dev_release(struct inode *inode, struct file *file)
 {
-        struct fuse_conn *fc;
-
-        spin_lock(&fuse_lock);
-        fc = file->private_data;
+        struct fuse_conn *fc = fuse_get_conn(file);
         if (fc) {
+                spin_lock(&fuse_lock);
                 fc->connected = 0;
                 end_requests(fc, &fc->pending);
                 end_requests(fc, &fc->processing);
-        }
-        spin_unlock(&fuse_lock);
-        if (fc) {
+                spin_unlock(&fuse_lock);
                 fasync_helper(-1, file, 0, &fc->fasync);
                 kobject_put(&fc->kobj);
         }
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 78700cbb9cdf..620579a69107 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -414,37 +414,6 @@ static struct fuse_conn *new_conn(void)
         return fc;
 }
 
-static struct fuse_conn *get_conn(struct file *file, struct super_block *sb)
-{
-        struct fuse_conn *fc;
-        int err;
-
-        err = -EINVAL;
-        if (file->f_op != &fuse_dev_operations)
-                goto out_err;
-
-        err = -ENOMEM;
-        fc = new_conn();
-        if (!fc)
-                goto out_err;
-
-        spin_lock(&fuse_lock);
-        err = -EINVAL;
-        if (file->private_data)
-                goto out_unlock;
-
-        kobject_get(&fc->kobj);
-        file->private_data = fc;
-        spin_unlock(&fuse_lock);
-        return fc;
-
- out_unlock:
-        spin_unlock(&fuse_lock);
-        kobject_put(&fc->kobj);
- out_err:
-        return ERR_PTR(err);
-}
-
 static struct inode *get_root_inode(struct super_block *sb, unsigned mode)
 {
         struct fuse_attr attr;
@@ -526,12 +495,9 @@ static void fuse_send_init(struct fuse_conn *fc)
 
 static unsigned long long conn_id(void)
 {
+        /* BKL is held for ->get_sb() */
         static unsigned long long ctr = 1;
-        unsigned long long val;
-        spin_lock(&fuse_lock);
-        val = ctr++;
-        spin_unlock(&fuse_lock);
-        return val;
+        return ctr++;
 }
 
 static int fuse_fill_super(struct super_block *sb, void *data, int silent)
@@ -556,10 +522,17 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
         if (!file)
                 return -EINVAL;
 
-        fc = get_conn(file, sb);
-        fput(file);
-        if (IS_ERR(fc))
-                return PTR_ERR(fc);
+        if (file->f_op != &fuse_dev_operations)
+                return -EINVAL;
+
+        /* Setting file->private_data can't race with other mount()
+           instances, since BKL is held for ->get_sb() */
+        if (file->private_data)
+                return -EINVAL;
+
+        fc = new_conn();
+        if (!fc)
+                return -ENOMEM;
 
         fc->flags = d.flags;
         fc->user_id = d.user_id;
@@ -589,10 +562,16 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
                 goto err_put_root;
 
         sb->s_root = root_dentry;
-        spin_lock(&fuse_lock);
         fc->mounted = 1;
         fc->connected = 1;
-        spin_unlock(&fuse_lock);
+        kobject_get(&fc->kobj);
+        file->private_data = fc;
+        /*
+         * atomic_dec_and_test() in fput() provides the necessary
+         * memory barrier for file->private_data to be visible on all
+         * CPUs after this
+         */
+        fput(file);
 
         fuse_send_init(fc);
 
@@ -601,6 +580,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
  err_put_root:
         dput(root_dentry);
  err:
+        fput(file);
         kobject_put(&fc->kobj);
         return err;
 }