Re: [PATCH v3] futex: Remove requirement for lock_page in

[Davidlohr Bueso <dave@stgolabs.net>]

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;
> }