linux-kernel - [PATCH V2] f2fs: Avoid double lock for cp

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Message-Id: <1587967204-24824-1-git-send-email-sayalil@codeaurora.org>
Date:   Mon, 27 Apr 2020 11:30:04 +0530
From:   Sayali Lokhande <sayalil@...eaurora.org>
To:     jaegeuk@...nel.org, yuchao0@...wei.com,
        linux-f2fs-devel@...ts.sourceforge.net
Cc:     stummala@...eaurora.org, linux-kernel@...r.kernel.org,
        Sayali Lokhande <sayalil@...eaurora.org>
Subject: [PATCH V2] f2fs: Avoid double lock for cp_rwsem during checkpoint

There could be a scenario where f2fs_sync_node_pages gets
called during checkpoint, which in turn tries to flush
inline data and calls iput(). This results in deadlock as
iput() tries to hold cp_rwsem, which is already held at the
beginning by checkpoint->block_operations().

Call stack :

Thread A		Thread B
f2fs_write_checkpoint()
- block_operations(sbi)
 - f2fs_lock_all(sbi);
  - down_write(&sbi->cp_rwsem);

                        - open()
                         - igrab()
                        - write() write inline data
                        - unlink()
- f2fs_sync_node_pages()
 - if (is_inline_node(page))
  - flush_inline_data()
   - ilookup()
     page = f2fs_pagecache_get_page()
     if (!page)
      goto iput_out;
     iput_out:

    - iput()
       iput(inode);
       - f2fs_evict_inode()
        - f2fs_truncate_blocks()
         - f2fs_lock_op()
           - down_read(&sbi->cp_rwsem);

Signed-off-by: Sayali Lokhande <sayalil@...eaurora.org>
---
 fs/f2fs/checkpoint.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index 5ba649e..97b6378 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -1219,21 +1219,19 @@ static int block_operations(struct f2fs_sb_info *sbi)
 		goto retry_flush_quotas;
 	}
 
-retry_flush_nodes:
 	down_write(&sbi->node_write);
 
 	if (get_pages(sbi, F2FS_DIRTY_NODES)) {
 		up_write(&sbi->node_write);
+		up_write(&sbi->node_change);
+		f2fs_unlock_all(sbi);
 		atomic_inc(&sbi->wb_sync_req[NODE]);
 		err = f2fs_sync_node_pages(sbi, &wbc, false, FS_CP_NODE_IO);
 		atomic_dec(&sbi->wb_sync_req[NODE]);
-		if (err) {
-			up_write(&sbi->node_change);
-			f2fs_unlock_all(sbi);
+		if (err)
 			goto out;
-		}
 		cond_resched();
-		goto retry_flush_nodes;
+		goto retry_flush_quotas;
 	}
 
 	/*
-- 
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