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152af11428
When sync() and link() are called concurrently, both threads may
enter hfs_bnode_find() without finding the node in the hash table
and proceed to create it.
Thread A:
hfsplus_write_inode()
-> hfsplus_write_system_inode()
-> hfs_btree_write()
-> hfs_bnode_find(tree, 0)
-> __hfs_bnode_create(tree, 0)
Thread B:
hfsplus_create_cat()
-> hfs_brec_insert()
-> hfs_bnode_split()
-> hfs_bmap_alloc()
-> hfs_bnode_find(tree, 0)
-> __hfs_bnode_create(tree, 0)
In this case, thread A creates the bnode, sets refcnt=1, and hashes it.
Thread B also tries to create the same bnode, notices it has already
been inserted, drops its own instance, and uses the hashed one without
getting the node.
```
node2 = hfs_bnode_findhash(tree, cnid);
if (!node2) { <- Thread A
hash = hfs_bnode_hash(cnid);
node->next_hash = tree->node_hash[hash];
tree->node_hash[hash] = node;
tree->node_hash_cnt++;
} else { <- Thread B
spin_unlock(&tree->hash_lock);
kfree(node);
wait_event(node2->lock_wq,
!test_bit(HFS_BNODE_NEW, &node2->flags));
return node2;
}
```
However, hfs_bnode_find() requires each call to take a reference.
Here both threads end up setting refcnt=1. When they later put the node,
this triggers:
BUG_ON(!atomic_read(&node->refcnt))
In this scenario, Thread B in fact finds the node in the hash table
rather than creating a new one, and thus must take a reference.
Fix this by calling hfs_bnode_get() when reusing a bnode newly created by
another thread to ensure the refcount is updated correctly.
A similar bug was fixed in HFS long ago in commit
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.