[bug report] btrfs: enable nowait async buffered writes

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Stefan Roesch,

The patch 8bf465b1c880: "btrfs: enable nowait async buffered writes"
from Sep 12, 2022, leads to the following Smatch static checker
warning:

	fs/btrfs/file.c:2113 btrfs_do_write_iter()
	warn: refcount leak 'inode->sync_writers.counter': lines='2113'

fs/btrfs/file.c
    2092 ssize_t btrfs_do_write_iter(struct kiocb *iocb, struct iov_iter *from,
    2093                             const struct btrfs_ioctl_encoded_io_args *encoded)
    2094 {
    2095         struct file *file = iocb->ki_filp;
    2096         struct btrfs_inode *inode = BTRFS_I(file_inode(file));
    2097         ssize_t num_written, num_sync;
    2098         const bool sync = iocb_is_dsync(iocb);
    2099 
    2100         /*
    2101          * If the fs flips readonly due to some impossible error, although we
    2102          * have opened a file as writable, we have to stop this write operation
    2103          * to ensure consistency.
    2104          */
    2105         if (BTRFS_FS_ERROR(inode->root->fs_info))
    2106                 return -EROFS;
    2107 
    2108         if (sync)
    2109                 atomic_inc(&inode->sync_writers);
    2110 
    2111         if (encoded) {
    2112                 if (iocb->ki_flags & IOCB_NOWAIT)
--> 2113                         return -EOPNOTSUPP;

This check used to happen before the atomic_inc(&inode->sync_writers);

    2114 
    2115                 num_written = btrfs_encoded_write(iocb, from, encoded);
    2116                 num_sync = encoded->len;
    2117         } else if (iocb->ki_flags & IOCB_DIRECT) {
    2118                 num_written = btrfs_direct_write(iocb, from);
    2119                 num_sync = num_written;
    2120         } else {
    2121                 num_written = btrfs_buffered_write(iocb, from);
    2122                 num_sync = num_written;
    2123         }
    2124 
    2125         btrfs_set_inode_last_sub_trans(inode);
    2126 
    2127         if (num_sync > 0) {
    2128                 num_sync = generic_write_sync(iocb, num_sync);
    2129                 if (num_sync < 0)
    2130                         num_written = num_sync;
    2131         }
    2132 
    2133         if (sync)
    2134                 atomic_dec(&inode->sync_writers);
    2135 
    2136         current->backing_dev_info = NULL;
    2137         return num_written;
    2138 }

regards,
dan carpenter