btrfs: nowait aio: Correct assignment of pos

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.

This fixes a user visible bug, as reported:

"apparently breaks several shell related features on my system.
In zsh history stopped working, because no new entries are added
anymore.
I fist noticed the issue when I tried to build mplayer. It uses a shell
script to generate a help_mp.h file:
[...]

Here is a simple testcase:

 % echo "foo" >> test
 % echo "foo" >> test
 % cat test
 foo
 %
"

Fixes: edf064e7c6fe ("btrfs: nowait aio support")
CC: Jens Axboe <axboe@kernel.dk>
Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Link: https://lkml.kernel.org/r/20170704042306.GA274@x4
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 59e2dcc..ad53832 100644 (file)
--- 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);