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: Xiong Zhou <xzhou@redhat.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
index 93b3ab0..f44f799 100644
--- a/fs/xfs/xfs_dir2_readdir.c
+++ b/fs/xfs/xfs_dir2_readdir.c
@@ -273,10 +273,11 @@
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 @@
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 @@
*/
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;