ext2: Silence lockdep warning about reclaim under xattr_sem

Lockdep complains about a chain:
  sb_internal#2 --> &ei->xattr_sem#2 --> fs_reclaim

and shrink_dentry_list -> ext2_evict_inode -> ext2_xattr_delete_inode ->
down_write(ei->xattr_sem) creating a locking cycle in the reclaim path.
This is however a false positive because when we are in
ext2_evict_inode() we are the only holder of the inode reference and
nobody else should touch xattr_sem of that inode. So we cannot ever
block on acquiring the xattr_sem in the reclaim path.

Silence the lockdep warning by using down_write_trylock() in
ext2_xattr_delete_inode() to not create false locking dependency.

Reported-by: "J. R. Okajima" <hooanon05g@gmail.com>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Jan Kara <jack@suse.cz>

diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c
index 0456bc990b5ee92c775af7d533aa327ff585b961..9ad07c7ef0b3fa31ea1a3552375b6638aee4c4d7 100644 (file)
--- a/fs/ext2/xattr.c
+++ b/fs/ext2/xattr.c
@@ -790,7 +790,15 @@ ext2_xattr_delete_inode(struct inode *inode)
        struct buffer_head *bh = NULL;
        struct ext2_sb_info *sbi = EXT2_SB(inode->i_sb);
 
-       down_write(&EXT2_I(inode)->xattr_sem);
+       /*
+        * We are the only ones holding inode reference. The xattr_sem should
+        * better be unlocked! We could as well just not acquire xattr_sem at
+        * all but this makes the code more futureproof. OTOH we need trylock
+        * here to avoid false-positive warning from lockdep about reclaim
+        * circular dependency.
+        */
+       if (WARN_ON_ONCE(!down_write_trylock(&EXT2_I(inode)->xattr_sem)))
+               return;
        if (!EXT2_I(inode)->i_file_acl)
                goto cleanup;