Re: d_splice_alias() problem.

Greg Banks
2004-05-04T10:23:37Z

G'day Dipankar,

I don't know if you've been following this thread on lkml, but Al Viro
wants an RCU expert's opinion on the following dcache patch.

Greg Banks wrote:
>
> > > * Dentry_stat.nr_unused can be be spuriously decremented when dput()
> > > races with __dget_unlocked(). Eventual result is nr_unused<0
> > > and kswapd loops. This is the problem I mentioned earlier. Note
> > > that this is not an NFS-specific problem. Fix is:
> > > [...first attempt elided...]
> 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.
>
> --- 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;
> }
>

viro@parcelfarce.linux.theplanet.co.uk wrote:
>
> a) ask RCU folks to review - the current logics in dcache.c is extremely
> brittle as it is.

Greg.
--
Greg Banks, R&D Software Engineer, SGI Australian Software Group.
I don't speak for SGI.