Re: [PATCH, RFC] Remove fasync() BKL usage, take 3325

Andrew Morton
2009-01-23T05:32:21Z

> On Thu, 22 Jan 2009 22:15:00 -0700 Jonathan Corbet <corbet@lwn.net> wrote:
> On Thu, 22 Jan 2009 06:51:04 -0800
> Andrew Morton <akpm@linux-foundation.org> wrote:
>
> > OK, replacing a lock_kernel() with a spin_lock(&global_lock) is pretty
> > straightforwad. But it's really really sad. It basically leaves a
> > great big FIXME in there. It'd be better to fix it.
> >
> > We don't have a handy lock in struct file which could be borrowed.
>
> Yeah, I noticed that too.
>
> > - We could add one
>
> The problem there is that this bloats struct file, and that seemed like
> something worth avoiding.

Not a big deal, really. There's one of these for each presently-open file.
It's not like dentries and inodes, which we cache after userspace has
closed off the file handles.

> It could easily be done, but I don't know
> why we would before knowing that the global spinlock is a problem.
>
> But... it's *already* protected by a global spinlock (the BKL) which is
> (still) more widely used.
>
> > - We could borrow file->f_path.dentry->d_inode->i_lock
>
> I didn't think of that one. Using a lock which is three indirections
> away seems a little obscure; again, I guess we could do that if the
> global spinlock actually turns out to be a problem.
>
> > - We could convert that field to long and use bitops (sounds nice?)
>
> I did think of that one. Reasons not to include growing struct file
> and the fact that there are places which set more than one flag at
> once. So we'd replace assignments with loops - and we still don't
> solve the fasync() problem.
>

I don't know what "the fasync() problem" is?