Re: [RFC PATCH] fs: elide the smp_rmb fence in fd_install()

[Mateusz Guzik]

On Thu, Dec 5, 2024 at 3:18 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Dec 05, 2024 at 01:03:32PM +0100, Mateusz Guzik wrote:
> >  void fd_install(unsigned int fd, struct file *file)
> >  {
> > -     struct files_struct *files = current->files;
> > +     struct files_struct *files;
> >       struct fdtable *fdt;
> >
> >       if (WARN_ON_ONCE(unlikely(file->f_mode & FMODE_BACKING)))
> >               return;
> >
> > +     /*
> > +      * Synchronized with expand_fdtable(), see that routine for an
> > +      * explanation.
> > +      */
> >       rcu_read_lock_sched();
> > +     files = READ_ONCE(current->files);
>
> What are you trying to do with that READ_ONCE()?  current->files
> itself is *not* changed by any of that code; current->files->fdtab is.

To my understanding this is the idiomatic way of spelling out the
non-existent in Linux smp_consume_load, for the resize_in_progress
flag.

Anyway to elaborate I'm gunning for a setup where the code is
semantically equivalent to having a lock around the work.
Pretend ->resize_lock exists, then:
fd_install:
files = current->files;
read_lock(files->resize_lock);
fdt = rcu_dereference_sched(files->fdt);
rcu_assign_pointer(fdt->fd[fd], file);
read_unlock(files->resize_lock);

expand_fdtable:
write_lock(files->resize_lock);
[snip]
rcu_assign_pointer(files->fdt, new_fdt);
write_unlock(files->resize_lock);

Except rcu_read_lock_sched + appropriately fenced resize_in_progress +
synchronize_rcu do it.

-- 
Mateusz Guzik <mjguzik gmail.com>