[PATCH v2] btrfs: Correct assignment of pos

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgold...@suse.com>

Assigning pos for usage early messes up in append mode, where
the pos is re-assigned in generic_write_checks(). Assign
pos later to get the correct position to write from iocb->ki_pos.

Since check_can_nocow also uses the value of pos, we shift
generic_write_checks() before check_can_nocow(). Checks with
IOCB_DIRECT are present in generic_write_checks(), so checking
for IOCB_NOWAIT is enough.

Also, put locking sequence in the fast path.

Changes since v1:
 - Moved pos higher up to encompass check_can_nocow() call.

Fixes: edf064e7c6fe ("btrfs: nowait aio support")
Signed-off-by: Goldwyn Rodrigues <rgold...@suse.com>
---
 fs/btrfs/file.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 59e2dccdf75b..ad53832838b5 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1875,16 +1875,25 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
        ssize_t num_written = 0;
        bool sync = (file->f_flags & O_DSYNC) || IS_SYNC(file->f_mapping->host);
        ssize_t err;
-       loff_t pos = iocb->ki_pos;
+       loff_t pos;
        size_t count = iov_iter_count(from);
        loff_t oldsize;
        int clean_page = 0;
 
-       if ((iocb->ki_flags & IOCB_NOWAIT) &&
-                       (iocb->ki_flags & IOCB_DIRECT)) {
-               /* Don't sleep on inode rwsem */
-               if (!inode_trylock(inode))
+       if (!inode_trylock(inode)) {
+               if (iocb->ki_flags & IOCB_NOWAIT)
                        return -EAGAIN;
+               inode_lock(inode);
+       }
+
+       err = generic_write_checks(iocb, from);
+       if (err <= 0) {
+               inode_unlock(inode);
+               return err;
+       }
+
+       pos = iocb->ki_pos;
+       if (iocb->ki_flags & IOCB_NOWAIT) {
                /*
                 * We will allocate space in case nodatacow is not set,
                 * so bail
@@ -1895,13 +1904,6 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
                        inode_unlock(inode);
                        return -EAGAIN;
                }
-       } else
-               inode_lock(inode);
-
-       err = generic_write_checks(iocb, from);
-       if (err <= 0) {
-               inode_unlock(inode);
-               return err;
        }
 
        current->backing_dev_info = inode_to_bdi(inode);
-- 
2.12.0