Re: [PATCH v2] fs/file.c: don't acquire files->file_lock in fd_install()

Al Viro
2015-06-22T02:32:27Z

On Tue, Apr 28, 2015 at 09:25:03PM -0700, Eric Dumazet wrote:

> @@ -553,11 +572,20 @@ void __fd_install(struct files_struct *files, unsigned int fd,
> struct file *file)
> {
> struct fdtable *fdt;
> - spin_lock(&files->file_lock);
> - fdt = files_fdtable(files);
> +
> + rcu_read_lock_sched();
> +
> + while (unlikely(files->resize_in_progress)) {
> + rcu_read_unlock_sched();
> + wait_event(files->resize_wait, !files->resize_in_progress);
> + rcu_read_lock_sched();
> + }
> + /* coupled with smp_wmb() in expand_fdtable() */
> + smp_rmb();
> + fdt = rcu_dereference_sched(files->fdt);
> BUG_ON(fdt->fd[fd] != NULL);
> rcu_assign_pointer(fdt->fd[fd], file);
> - spin_unlock(&files->file_lock);
> + rcu_read_unlock_sched();

Umm... You've taken something that was safe to use in atomic contexts
and turned into something that might wait for GFP_KERNEL allocation; what's
to guarantee that no users get broken by that? At the very least, you want
to slap might_sleep() in there - the actual sleep is going to be very rare,
so it would be an extremely hard to reproduce and debug.

AFAICS, all current in-tree users should be safe, but fd_install() is exported
and quiet changes of that sort are rather antisocial. Generally I don't give
a damn about out-of-tree code, but this one is over the top.

I _think_ it's otherwise OK, but please, add might_sleep() *AND* a note in
Documentation/filesystems/porting.

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