Re: [PATCH 05/11] fs: add iterate_supers_excl() and iterate_supers_reverse_excl()

[Dave Chinner <david@fromorbit.com>]

On Thu, Nov 30, 2017 at 12:48:15AM +0100, Rafael J. Wysocki wrote:
> On Thu, Nov 30, 2017 at 12:23 AM, Luis R. Rodriguez <mcgrof@kernel.org> wrote:
> > There are use cases where we wish to traverse the superblock list
> > but also capture errors, and in which case we want to avoid having
> > our callers issue a lock themselves since we can do the locking for
> > the callers. Provide a iterate_supers_excl() which calls a function
> > with the write lock held. If an error occurs we capture it and
> > propagate it.
> >
> > Likewise there are use cases where we wish to traverse the superblock
> > list but in reverse order. The new iterate_supers_reverse_excl() helpers
> > does this but also also captures any errors encountered.
> >
> > Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
> > ---
> >  fs/super.c         | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/fs.h |  2 ++
> >  2 files changed, 93 insertions(+)
> >
> > diff --git a/fs/super.c b/fs/super.c
> > index a63513d187e8..885711c1d35b 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -605,6 +605,97 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
> >         spin_unlock(&sb_lock);
> >  }
> >
> > +/**
> > + *     iterate_supers_excl - exclusively call func for all active superblocks
> > + *     @f: function to call
> > + *     @arg: argument to pass to it
> > + *
> > + *     Scans the superblock list and calls given function, passing it
> > + *     locked superblock and given argument. Returns 0 unless an error
> > + *     occurred on calling the function on any superblock.
> > + */
> > +int iterate_supers_excl(int (*f)(struct super_block *, void *), void *arg)
> > +{
> > +       struct super_block *sb, *p = NULL;
> > +       int error = 0;
> > +
> > +       spin_lock(&sb_lock);
> > +       list_for_each_entry(sb, &super_blocks, s_list) {
> > +               if (hlist_unhashed(&sb->s_instances))
> > +                       continue;
> > +               sb->s_count++;
> > +               spin_unlock(&sb_lock);
> 
> Can anything bad happen if the list is modified at this point by a
> concurrent thread?

No. We have a valid reference to sb->s_count and that keeps it on
the list while we have the lock dropped. The sb reference isn't
dropped until we've iterated to the next sb on the list and taken a
reference to that, hence it's safe to drop and regain the list lock
without needing to restart the iteration.

> > +
> > +               down_write(&sb->s_umount);
> > +               if (sb->s_root && (sb->s_flags & SB_BORN)) {
> > +                       error = f(sb, arg);
> > +                       if (error) {
> > +                               up_write(&sb->s_umount);
> > +                               spin_lock(&sb_lock);
> > +                               __put_super(sb);
> > +                               break;
> > +                       }
> > +               }
> > +               up_write(&sb->s_umount);
> > +
> > +               spin_lock(&sb_lock);
> > +               if (p)
> > +                       __put_super(p);
> > +               p = sb;

This code here is what drops the reference to the previous sb
we've iterated past.

FWIW, this "hold until next is held" iteration pattern is used
frequently for inodes, dentries, and other reference counted VFS
objects so we can iterate the list without needing to hold the
list lock for the entire iteration....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com