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

Eric Dumazet
2015-04-16T20:42:12Z

On Thu, 2015-04-16 at 19:09 +0100, Al Viro wrote:
> On Thu, Apr 16, 2015 at 02:16:31PM +0200, Mateusz Guzik wrote:
> > @@ -165,8 +165,10 @@ static int expand_fdtable(struct files_struct *files, int nr)
> > cur_fdt = files_fdtable(files);
> > if (nr >= cur_fdt->max_fds) {
> > /* Continue as planned */
> > + write_seqcount_begin(&files->fdt_seqcount);
> > copy_fdtable(new_fdt, cur_fdt);
> > rcu_assign_pointer(files->fdt, new_fdt);
> > + write_seqcount_end(&files->fdt_seqcount);
> > if (cur_fdt != &files->fdtab)
> > call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
>
> Interesting. AFAICS, your test doesn't step anywhere near that path,
> does it? So basically you never hit the retries during that...

Right, but then the table is almost never changed for a given process,
as we only increase it by power of two steps.

(So I scratch my initial comment, fdt_seqcount is really mostly read)