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

[Kirill Korotaev <dev@sw.ru>]

Neil,

> 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.
you are right... :/
And this is actually why we originally inserted check for race
in select_parent() under dcache_lock... I just lost my memory :(

> 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.
Can try...

Thanks,
Kirill

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