Re: d_splice_alias() problem.

[Neil Brown]

On Tuesday May 4, gnb@xxxxxxxxxxxxxxxxx wrote:
> > >
> > > --- linux.orig/fs/dcache.c    Mon May  3 21:46:30 2004
> > > +++ linux/fs/dcache.c Mon May  3 21:49:07 2004
> > > @@ -255,8 +255,8 @@
> > >
> > >  static inline struct dentry * __dget_locked(struct dentry *dentry)
> > >  {
> > > -     atomic_inc(&dentry->d_count);
> > > -     if (atomic_read(&dentry->d_count) == 1) {
> > > +     if (atomic_inc(&dentry->d_count) == 1) {
> > 
> > One problem with this is that (in include/asm-i386/atomic.h at least):
> >   static __inline__ void atomic_inc(atomic_t *v)
> 
> Ok, how about this...it's portable, and not racy, but may perturb the
> logic slightly by also taking dentries off the unused list in the case
> where they already had d_count>=1.  I'm not sure how significant that is.
> In any case this also passes my tests.

I think this patch is good and needed.

I think the race can happen if:
       dentry->d_count == 1, not on list

   thread 1                       thread 2
   enter __dget_locked            enter dput
   atomic_inc(d_count) (now 2)
                                  atomic_dec_and_lock(d_count...) (now 1)
   if(atomic_read(d_count)==1 ....
         remove from list


This will remove it from the unused list when it isn't
on, and will decrement nr_unused which, as you say, is bad.

I don't think there can be any problem with taking dentries with a
non-zero d_count off the unused_list randomly (providing dcache_lock
is held of course).

The comment at the top of d_lookup says that it doesn't take dentries
off the unused_list, but instead leaves that task to assorted other
code.

prune_dcache and shrink_dcache_sb and select_parent will all quietly
remove such dentries from the unused list and there is no reason that
__dget_locked shouldn't aswell.  Possibly that patch should update the 
d_lookup comment to add __dget_locked to the set of functions that
clean up after it.

NeilBrown

> 
> 
> --- linux.orig/fs/dcache.c	Mon May  3 21:46:30 2004
> +++ linux/fs/dcache.c	Tue May  4 14:34:44 2004
> @@ -256,7 +256,7 @@
>  static inline struct dentry * __dget_locked(struct dentry *dentry)
>  {
>  	atomic_inc(&dentry->d_count);
> -	if (atomic_read(&dentry->d_count) == 1) {
> +	if (!list_empty(&dentry->d_lru)) {
>  		dentry_stat.nr_unused--;
>  		list_del_init(&dentry->d_lru);
>  	}
> @@ -663,6 +663,7 @@
>  		if (gfp_mask & __GFP_FS)
>  			prune_dcache(nr);
>  	}
> +	BUG_ON(dentry_stat.nr_unused < 0);
>  	return dentry_stat.nr_unused;
>  }
>  
> 
> 
> Greg.
> -- 
> Greg Banks, R&D Software Engineer, SGI Australian Software Group.
> I don't speak for SGI.
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