vfs: fix race between evice_inodes() and find_inode()&iput()

Hi, all

Recently I noticed a bug[1] in btrfs, after digged it into
and I believe it'a race in vfs.

Let's assume there's a inode (ie ino 261) with i_count 1 is
called by iput(), and there's a concurrent thread calling
generic_shutdown_super().

cpu0:                              cpu1:
iput() // i_count is 1
  ->spin_lock(inode)
  ->dec i_count to 0
  ->iput_final()                    generic_shutdown_super()
    ->__inode_add_lru()               ->evict_inodes()
      // cause some reason[2]           ->if (atomic_read(inode->i_count)) continue;
      // return before                  // inode 261 passed the above check
      // list_lru_add_obj()             // and then schedule out
   ->spin_unlock()
// note here: the inode 261
// was still at sb list and hash list,
// and I_FREEING|I_WILL_FREE was not been set

btrfs_iget()
  // after some function calls
  ->find_inode()
    // found the above inode 261
    ->spin_lock(inode)
   // check I_FREEING|I_WILL_FREE
   // and passed
      ->__iget()
    ->spin_unlock(inode)                // schedule back
                                        ->spin_lock(inode)
                                        // check (I_NEW|I_FREEING|I_WILL_FREE) flags,
                                        // passed and set I_FREEING
iput()                                  ->spin_unlock(inode)
  ->spin_lock(inode)			  ->evict()
  // dec i_count to 0
  ->iput_final()
    ->spin_unlock()
    ->evict()

Now, we have two threads simultaneously evicting
the same inode, which may trigger the BUG(inode->i_state & I_CLEAR)
statement both within clear_inode() and iput().

To fix the bug, recheck the inode->i_count after holding i_lock.
Because in the most scenarios, the first check is valid, and
the overhead of spin_lock() can be reduced.

If there is any misunderstanding, please let me know, thanks.

[1]: https://lore.kernel.org/linux-btrfs/000000000000eabe1d0619c48986@google.com/
[2]: The reason might be 1. SB_ACTIVE was removed or 2. mapping_shrinkable()
return false when I reproduced the bug.

Reported-by: syzbot+67ba3c42bcbb4665d3ad@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=67ba3c42bcbb4665d3ad
CC: stable@vger.kernel.org
Fixes: 63997e98a3 ("split invalidate_inodes()")
Signed-off-by: Julian Sun <sunjunchao2870@gmail.com>
Link: https://lore.kernel.org/r/20240823130730.658881-1-sunjunchao2870@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/inode.c

@@ -771,6 +771,10 @@ again:
 			continue;
 
 		spin_lock(&inode->i_lock);
+		if (atomic_read(&inode->i_count)) {
+			spin_unlock(&inode->i_lock);
+			continue;
+		}
 		if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {
 			spin_unlock(&inode->i_lock);
 			continue;