On Wed, Sep 29, 2010 at 09:52:53PM -0700, Andrew Morton wrote:
> On Wed, 29 Sep 2010 22:18:39 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>
> > From: Nick Piggin <npiggin@xxxxxxx>
> >
> > The inode use statistics are currently protected by the inode_lock.
> > Before we can remove the inode_lock, we need to protect these
> > counters against races. Do this by converting them to atomic
> > counters so they ar enot dependent on any lock at all.
>
> typo
>
> >
> > ...
> >
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -764,7 +764,8 @@ static long wb_check_old_data_flush(struct bdi_writeback *wb)
> > wb->last_old_flush = jiffies;
> > nr_pages = global_page_state(NR_FILE_DIRTY) +
> > global_page_state(NR_UNSTABLE_NFS) +
> > - (inodes_stat.nr_inodes - inodes_stat.nr_unused);
> > + (atomic_read(&inodes_stat.nr_inodes) -
> > + atomic_read(&inodes_stat.nr_unused));
>
> race bug.
What new race? The code has gone from using subtraction of unlocked
counters to using subtraction of unlocked atomic counters. I can't
see any new race condition in that transformation....
> > +struct inodes_stat_t {
> > + atomic_t nr_inodes;
> > + atomic_t nr_unused;
> > + int dummy[5]; /* padding for sysctl ABI compatibility */
> > +};
>
> OK, that's a hack. The first two "ints" are copied out to userspace.
> This change assumes that sizeof(atomic_t)=4 and that an atomic_t has
> the same layout, alignment and padding as an int.
>
> Probably that's true in current kernels and with current architectures
> but it's a hack and it's presumptive.
>
> It shouldn't be snuck into the tree unchangelogged and uncommented.
>
> (time passes)
>
> OK, I see that all of this gets reverted later on. Please update the
> changelog so the next reviewer doesn't get fooled.
That's the multiple changes to the counter infrastructure I
described in the preliminary series description. clearly it needs to
be cleaned up.
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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