vfs: grab the lock instead of blocking in __fd_install during resizing

Explicit locking in the fallback case provides a safe state of the
table. Getting rid of blocking semantics makes __fd_install usable
again in non-sleepable contexts, which easies backporting efforts.

There is a side effect of slightly nicer assembly for the common case
as might_sleep can now be removed.
Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/porting

@@ -501,10 +501,6 @@ in your dentry operations instead.
 	is non-NULL.  Note that link body isn't available anymore, so if you need it,
 	store it as cookie.
 --
-[mandatory]
-	__fd_install() & fd_install() can now sleep. Callers should not
-	hold a spinlock	or other resources that do not allow a schedule.
---
 [mandatory]
 	any symlink that might use page_follow_link_light/page_put_link() must
 	have inode_nohighmem(inode) called before anything might start playing with

fs/file.c

@@ -592,13 +592,16 @@ void __fd_install(struct files_struct *files, unsigned int fd,
 {
 	struct fdtable *fdt;
 
-	might_sleep();
 	rcu_read_lock_sched();
 
-	while (unlikely(files->resize_in_progress)) {
+	if (unlikely(files->resize_in_progress)) {
 		rcu_read_unlock_sched();
-		wait_event(files->resize_wait, !files->resize_in_progress);
-		rcu_read_lock_sched();
+		spin_lock(&files->file_lock);
+		fdt = files_fdtable(files);
+		BUG_ON(fdt->fd[fd] != NULL);
+		rcu_assign_pointer(fdt->fd[fd], file);
+		spin_unlock(&files->file_lock);
+		return;
 	}
 	/* coupled with smp_wmb() in expand_fdtable() */
 	smp_rmb();