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

Eric Dumazet
2015-06-23T05:31:36Z

On Mon, 2015-06-22 at 03:32 +0100, Al Viro wrote:
> 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.
>

Good points. I am currently traveling and will address this asap.

Thanks

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