[PATCH] erofs: Deadlock caused by kswap work in low memory scenarios

[Huang Jianan]

在 2021/11/23 10:59, Jianhua1 Hao 郝建华 via Linux-erofs 写道:
> *We also found that it is easy to cause deadlock in the kswap scene， 
> We observed the following deadlock in the stress test under low memory 
> scenario，****Same as "erofs: fix deadlock when shrink erofs slab".*
> **
>
> Thread A: Thread B:
>
> erofs_try_to_release_workgroup(grp = 
> 0xFFFFFF87ADFEE610)erofs_insert_workgroup()
>
> erofs_workgroup_try_to_freeze(grp, 1)//xa lock is held here
>
> //set ref count to EROFS_LOCKED_MAGICxa_lock(&sbi->managed_pslots);
>
> atomic_cmpxchg(&grp->refcount, val,EROFS_LOCKED_MAGIC)pre = 
> __xa_cmpxchg(&sbi->managed_pslots, grp->index, NULL, grp, GFP_NOFS);
>
> xa_erase(&sbi->managed_pslots, grp->index)erofs_workgroup_get(pre)  
> //pre = grp = 0xFFFFFF87ADFEE610
>
> //stuck there to wait for xa lock, already held by thread 
> Berofs_wait_on_workgroup_freezed(grp);
>
> xa_lock(xa); //wait ref count to be unlocked, which should be done by 
> thread A
>
> atomic_cond_read_relaxed(&grp->refcount, VAL != EROFS_LOCKED_MAGIC);
>
> Follow-up fix:it need to hold the xa lock before freeze the workgroup
>
> beacuse we will operate xarry?
>
Hi,  JianHua,

The fix is in the patch, please test it kindly if you have condition.
https://lore.kernel.org/linux-erofs/YZcJpDs3FKpSfzAE@B-P7TQMD6M-0146/T/#t

---
  fs/erofs/utils.c | 9 +++++++--
  1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/erofs/utils.c b/fs/erofs/utils.c
index 84da2c280012..84a59f075dd1 100644
--- a/fs/erofs/utils.c
+++ b/fs/erofs/utils.c
@@ -150,7 +150,7 @@ 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);
@@ -165,15 +165,20 @@ 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;
  }


Thanks,
Jianan

>
> ------------------------------------------------------------------------
> Jianhua1 Hao
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