[PATCH 5.15 117/179] erofs: fix deadlock when shrink erofs slab

[Greg Kroah-Hartman]

From: Huang Jianan <huangjianan@xxxxxxxx>

[ Upstream commit 57bbeacdbee72a54eb97d56b876cf9c94059fc34 ]

We observed the following deadlock in the stress test under low
memory scenario:

Thread A                               Thread B
- erofs_shrink_scan
 - erofs_try_to_release_workgroup
  - erofs_workgroup_try_to_freeze -- A
                                       - z_erofs_do_read_page
                                        - z_erofs_collection_begin
                                         - z_erofs_register_collection
                                          - erofs_insert_workgroup
                                           - xa_lock(&sbi->managed_pslots) -- B
                                           - erofs_workgroup_get
                                            - erofs_wait_on_workgroup_freezed -- A
  - xa_erase
   - xa_lock(&sbi->managed_pslots) -- B

To fix this, it needs to hold xa_lock before freezing the workgroup
since xarray will be touched then. So let's hold the lock before
accessing each workgroup, just like what we did with the radix tree
before.

[ Gao Xiang: Jianhua Hao also reports this issue at
  https://lore.kernel.org/r/b10b85df30694bac8aadfe43537c897a@xxxxxxxxxx ]

Link: https://lore.kernel.org/r/20211118135844.3559-1-huangjianan@xxxxxxxx
Fixes: 64094a04414f ("erofs: convert workstn to XArray")
Reviewed-by: Chao Yu <chao@xxxxxxxxxx>
Reviewed-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
Signed-off-by: Huang Jianan <huangjianan@xxxxxxxx>
Reported-by: Jianhua Hao <haojianhua1@xxxxxxxxxx>
Signed-off-by: Gao Xiang <xiang@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/erofs/utils.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/erofs/utils.c b/fs/erofs/utils.c
index bd86067a63f7f..3ca703cd5b24a 100644
--- a/fs/erofs/utils.c
+++ b/fs/erofs/utils.c
@@ -141,7 +141,7 @@ static bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
 	 * however in order to avoid some race conditions, add a
 	 * DBG_BUGON to observe this in advance.
 	 */
-	DBG_BUGON(xa_erase(&sbi->managed_pslots, grp->index) != grp);
+	DBG_BUGON(__xa_erase(&sbi->managed_pslots, grp->index) != grp);
 
 	/* last refcount should be connected with its managed pslot.  */
 	erofs_workgroup_unfreeze(grp, 0);
@@ -156,15 +156,19 @@ static unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
 	unsigned int freed = 0;
 	unsigned long index;
 
+	xa_lock(&sbi->managed_pslots);
 	xa_for_each(&sbi->managed_pslots, index, grp) {
 		/* try to shrink each valid workgroup */
 		if (!erofs_try_to_release_workgroup(sbi, grp))
 			continue;
+		xa_unlock(&sbi->managed_pslots);
 
 		++freed;
 		if (!--nr_shrink)
-			break;
+			return freed;
+		xa_lock(&sbi->managed_pslots);
 	}
+	xa_unlock(&sbi->managed_pslots);
 	return freed;
 }
 
-- 
2.33.0