anon-inodes use open coded atomic_inc for the shared inode

Since we know the shared inode count is always >0, we can avoid igrab()
and use an open coded atomic_inc().

This also fixes a bug noticed by Yan Zheng <yanzheng@21cn.com>: were checking
for an IS_ERR() return from igrab(), but it actually returns NULL on error.

Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Cc: Yan Zheng <yanzheng@21cn.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 13 deletions

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index b4a75880f6fd..23321889d9b0 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -76,7 +76,6 @@ int anon_inode_getfd(int *pfd, struct inode **pinode, struct file **pfile,
 {
         struct qstr this;
         struct dentry *dentry;
-        struct inode *inode;
         struct file *file;
         int error, fd;
 
@@ -86,15 +85,9 @@ int anon_inode_getfd(int *pfd, struct inode **pinode, struct file **pfile,
         if (!file)
                 return -ENFILE;
 
-        inode = igrab(anon_inode_inode);
-        if (IS_ERR(inode)) {
-                error = PTR_ERR(inode);
-                goto err_put_filp;
-        }
-
         error = get_unused_fd();
         if (error < 0)
-                goto err_iput;
+                goto err_put_filp;
         fd = error;
 
         /*
@@ -108,14 +101,22 @@ int anon_inode_getfd(int *pfd, struct inode **pinode, struct file **pfile,
         dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
         if (!dentry)
                 goto err_put_unused_fd;
+
+        /*
+         * We know the anon_inode inode count is always greater than zero,
+         * so we can avoid doing an igrab() and we can use an open-coded
+         * atomic_inc().
+         */
+        atomic_inc(&anon_inode_inode->i_count);
+
         dentry->d_op = &anon_inodefs_dentry_operations;
         /* Do not publish this dentry inside the global dentry hash table */
         dentry->d_flags &= ~DCACHE_UNHASHED;
-        d_instantiate(dentry, inode);
+        d_instantiate(dentry, anon_inode_inode);
 
         file->f_path.mnt = mntget(anon_inode_mnt);
         file->f_path.dentry = dentry;
-        file->f_mapping = inode->i_mapping;
+        file->f_mapping = anon_inode_inode->i_mapping;
 
         file->f_pos = 0;
         file->f_flags = O_RDWR;
@@ -127,14 +128,12 @@ int anon_inode_getfd(int *pfd, struct inode **pinode, struct file **pfile,
         fd_install(fd, file);
 
         *pfd = fd;
-        *pinode = inode;
+        *pinode = anon_inode_inode;
         *pfile = file;
         return 0;
 
 err_put_unused_fd:
         put_unused_fd(fd);
-err_iput:
-        iput(inode);
 err_put_filp:
         put_filp(file);
         return error;