[PATCH 1/1] btrfs: file_remove_privs needs an exclusive lock

[Bernd Schubert <bschubert@ddn.com>]

file_remove_privs might call into notify_change(), which
requires to hold an exclusive lock.

Fixes: e9adabb9712e ("btrfs: use shared lock for direct writes within EOF")
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Goldwyn Rodrigues <rgoldwyn@suse.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Dharmendra Singh <dsingh@ddn.com>
Cc: David Sterba <dsterba@suse.com>
Cc: linux-btrfs@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
---
 fs/btrfs/file.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index fd03e689a6be..c4b304a2948e 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1466,8 +1466,12 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	if (iocb->ki_flags & IOCB_NOWAIT)
 		ilock_flags |= BTRFS_ILOCK_TRY;
 
-	/* If the write DIO is within EOF, use a shared lock */
-	if (iocb->ki_pos + iov_iter_count(from) <= i_size_read(inode))
+	/* If the write DIO is within EOF, use a shared lock and also only
+	 * if security bits will likely not be dropped. Either will need
+	 * to be rechecked after the lock was acquired.
+	 */
+	if (iocb->ki_pos + iov_iter_count(from) <= i_size_read(inode) &&
+	    IS_NOSEC(inode))
 		ilock_flags |= BTRFS_ILOCK_SHARED;
 
 relock:
@@ -1475,6 +1479,12 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	if (err < 0)
 		return err;
 
+	if (ilock_flags & BTRFS_ILOCK_SHARED && !IS_NOSEC(inode)) {
+		btrfs_inode_unlock(BTRFS_I(inode), ilock_flags);
+		ilock_flags &= ~BTRFS_ILOCK_SHARED;
+		goto relock;
+	}
+
 	err = generic_write_checks(iocb, from);
 	if (err <= 0) {
 		btrfs_inode_unlock(BTRFS_I(inode), ilock_flags);
-- 
2.39.2