Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install

Eric Dumazet
2015-04-17T21:47:02Z

On Thu, 2015-04-16 at 14:16 +0200, Mateusz Guzik wrote:
> Hi,
>
> Currently obtaining a new file descriptor results in locking fdtable
> twice - once in order to reserve a slot and second time to fill it

...

> void __fd_install(struct files_struct *files, unsigned int fd,
> struct file *file)
> {
> + unsigned long seq;

unsigned int seq;

> struct fdtable *fdt;
> - 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);
> +
> + rcu_read_lock();
> + do {
> + seq = read_seqcount_begin(&files->fdt_seqcount);
> + fdt = files_fdtable_seq(files);
> + /*
> + * Entry in the table can already be equal to file if we
> + * had to restart and copy_fdtable picked up our update.
> + */
> + BUG_ON(!(fdt->fd[fd] == NULL || fdt->fd[fd] == file));
> + rcu_assign_pointer(fdt->fd[fd], file);
> + smp_mb();
> + } while (__read_seqcount_retry(&files->fdt_seqcount, seq));
> + rcu_read_unlock();
> }
>

So one problem here is :

As soon as rcu_assign_pointer(fdt->fd[fd], file) is done, and other cpu
does one expand_fdtable() and releases files->file_lock, another cpu can
close(fd).

Then another cpu can reuse the [fd] now empty slot and install a new
file in it.

Then this cpu will crash here :

BUG_ON(!(fdt->fd[fd] == NULL || fdt->fd[fd] == file));