On Tue, 19 Jan 2016, Bueso wrote:
/*
* Private mappings are handled in a simple way.
*
+ * If the futex key is stored on an anonymous page, then the associated
+ * object is the mm which is implicitly pinned by the calling process.
+ *
* NOTE: When userspace waits on a MAP_SHARED mapping, even if
* it's a read-only handle, it's expected that futexes attach to
* the object not the particular process.
@@ -566,16 +592,61 @@ again:
key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
key->private.mm = mm;
key->private.address = address;
+
+ get_futex_key_refs(key); /* implies MB (B) */
+
} else {
+ struct inode *inode;
+
+ /*
+ * The associtated futex object in this case is the inode and
+ * the page->mapping must be traversed. Ordinarily this should
+ * be stabilised under page lock but it's not strictly
+ * necessary in this case as we just want to pin the inode, not
+ * update radix tree or anything like that.
+ *
+ * The RCU read lock is taken as the inode is finally freed
+ * under RCU. If the mapping still matches expectations then the
+ * mapping->host can be safely accessed as being a valid inode.
+ */
+ rcu_read_lock();
+ if (READ_ONCE(page->mapping) != mapping ||
+ !mapping->host) {
+ rcu_read_unlock();
+ put_page(page);
+
+ goto again;
+ }
+ inode = READ_ONCE(mapping->host);
+
+ /*
+ * Take a reference unless it is about to be freed. Previously
+ * this reference was taken by ihold under the page lock
+ * pinning the inode in place so i_lock was unnecessary. The
+ * only way for this check to fail is if the inode was
+ * truncated in parallel so warn for now if this happens.
+ *
+ * We are not calling into get_futex_key_refs() in file-backed
+ * cases, therefore a successful atomic_inc return below will
+ * guarantee that get_futex_key() will continue to imply MB (B).
+ */
+ if (WARN_ON_ONCE(!atomic_inc_not_zero(&inode->i_count))) {
+ rcu_read_unlock();
+ put_page(page);
+
+ goto again;
+ }
+
+ /* Should be impossible but lets be paranoid for now */
+ BUG_ON(inode->i_mapping != mapping);
Hmm, do we want to transform this into an if and do rcu unlock and then just
call BUG()? I't doesn't matter at this point _anyway_, but it would be the right
thing to do, no?
+
key->both.offset |= FUT_OFF_INODE; /* inode-based key */
- key->shared.inode = mapping->host;
+ key->shared.inode = inode;
key->shared.pgoff = basepage_index(page);
+ rcu_read_unlock();
}
- get_futex_key_refs(key); /* implies MB (B) */
-
out:
- unlock_page(page);
put_page(page);
return err;
}
