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Commit 9f541801 authored by Dave Chinner's avatar Dave Chinner Committed by Al Viro
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xfs: concurrent readdir hangs on data buffer locks 



There's a three-process deadlock involving shared/exclusive barriers
and inverted lock orders in the directory readdir implementation.
It's a pre-existing problem with lock ordering, exposed by the
VFS parallelisation code.

process 1               process 2               process 3
---------               ---------               ---------
readdir
iolock(shared)
  get_leaf_dents
    iterate entries
       ilock(shared)
       map, lock and read buffer
       iunlock(shared)
       process entries in buffer
       .....
                                                readdir
                                                iolock(shared)
                                                  get_leaf_dents
                                                    iterate entries
                                                      ilock(shared)
                                                      map, lock buffer
                                                      <blocks>
                        finish ->iterate_shared
                        file_accessed()
                          ->update_time
                            start transaction
                            ilock(excl)
                            <blocks>
        .....
        finishes processing buffer
        get next buffer
          ilock(shared)
          <blocks>

And that's the deadlock.

Fix this by dropping the current buffer lock in process 1 before
trying to map the next buffer. This means we keep the lock order of
ilock -> buffer lock intact and hence will allow process 3 to make
progress and drop it's ilock(shared) once it is done.

Reported-by: default avatarXiong Zhou <xzhou@redhat.com>
Signed-off-by: default avatarDave Chinner <dchinner@redhat.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent fe742fd4

fs/xfs/xfs_dir2_readdir.c

+14 −9
Original line number Diff line number Diff line
@@ -273,10 +273,11 @@ xfs_dir2_leaf_readbuf( 
	size_t			bufsize,

	struct xfs_dir2_leaf_map_info *mip,

	xfs_dir2_off_t		*curoff,

	struct xfs_buf		**bpp)

	struct xfs_buf		**bpp,

	bool			trim_map)

{

	struct xfs_inode	*dp = args->dp;

	struct xfs_buf		*bp = *bpp;

	struct xfs_buf		*bp = NULL;

	struct xfs_bmbt_irec	*map = mip->map;

	struct blk_plug		plug;

	int			error = 0;
@@ -286,13 +287,10 @@ xfs_dir2_leaf_readbuf( 
	struct xfs_da_geometry	*geo = args->geo;



	/*

	 * If we have a buffer, we need to release it and

	 * take it out of the mapping.

	 * If the caller just finished processing a buffer, it will tell us

	 * we need to trim that block out of the mapping now it is done.

	 */



	if (bp) {

		xfs_trans_brelse(NULL, bp);

		bp = NULL;

	if (trim_map) {

		mip->map_blocks -= geo->fsbcount;

		/*

		 * Loop to get rid of the extents for the
@@ -533,10 +531,17 @@ xfs_dir2_leaf_getdents( 
		 */

		if (!bp || ptr >= (char *)bp->b_addr + geo->blksize) {

			int	lock_mode;

			bool	trim_map = false;



			if (bp) {

				xfs_trans_brelse(NULL, bp);

				bp = NULL;

				trim_map = true;

			}



			lock_mode = xfs_ilock_data_map_shared(dp);

			error = xfs_dir2_leaf_readbuf(args, bufsize, map_info,

						      &curoff, &bp);

						      &curoff, &bp, trim_map);

			xfs_iunlock(dp, lock_mode);

			if (error || !map_info->map_valid)

				break;