Re: [PATCH] Fix shrink_dcache_parent() against shrink_dcache_memory() race (3rd updated patch)]

[Neil Brown <neilb@suse.de>]

On Thursday March 9, dev@openvz.org wrote:
> Andrew,
> 
> Acked-By: Kirill Korotaev <dev@openvz.org>

I'm afraid that I'm not convinced.

> > +static int wait_on_prunes(struct super_block *sb)
> > +{
> > +	DEFINE_WAIT(wait);
> > +	int prunes_remaining = 0;
> > +
> > +#ifdef DCACHE_DEBUG
> > +	printk(KERN_DEBUG "%s: waiting for %d prunes\n", __FUNCTION__,
> > +	       sb->s_prunes);
> > +#endif
> > +
> > +	spin_lock(&dcache_lock);
> > +	for (;;) {
> > +		prepare_to_wait(&sb->s_wait_prunes, &wait,
> > +				TASK_UNINTERRUPTIBLE);
> > +		if (!sb->s_prunes)
> > +			break;
> > +		spin_unlock(&dcache_lock);
> > +		schedule();
> > +		prunes_remaining = 1;
> > +		spin_lock(&dcache_lock);
> > +	}
> > +	spin_unlock(&dcache_lock);
> > +	finish_wait(&sb->s_wait_prunes, &wait);
> > +	return prunes_remaining;
> > +}

I don't think that a return value from wait_on_prunes is meaningful.
All it tells us is whether a prune_one_dentry finished before or after
wait_on_prunes takes the spin_lock.  This isn't very useful
information as it has no significance to upper levels.

So:

> > +		do {
> > +			shrink_dcache_parent(root);
> > +		} while(wait_on_prunes(sb));
> > +

Suppose shrink_dcache_parent misses on dentry because the inode was being
iput.  This iput completes immediately that
shrink_dcache_parent completes.  It decrements ->s_prunes and when
wait_on_prunes takes dcache_lock, ->s_prunes is zero so the loop
terminates, and the remaining dentries - the parents of the dentry
what was undergoing iput - don't get put.

I really think that we need to stop prune_one_dentry from being called
on dentries for a filesystem that is being unmounted.  With that code
currently in -git, that means passing a 'struct super_block *' into
prune_dcache so that it ignores any filesystem with ->s_root==NULL
unless that filesystem is the filesystem that was passed.

NeilBrown
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