Re: [PATCH] shrink_dcache_parent() races against shrink_dcache_memory()

[Kirill Korotaev <dev@sw.ru>]

> > Kirill Korotaev <dev@sw.ru> discovered a race between shrink_dcache_parent()
> > and shrink_dcache_memory(). That one is based on dput() is calling
> > dentry_iput() too early and therefore is giving up the dcache_lock. This leads
> > to the situation that the parent dentry might be still referenced although all
> > childs are already dead. This parent is ignore by a concurrent select_parent()
> > call which might be the reason for busy inode after umount failures.
> >
> > This is from Kirill's original patch:
> >
> > CPU 1                    CPU 2
> > ~~~~~                    ~~~~~
> > umount /dev/sda1
> > generic_shutdown_super   shrink_dcache_memory
> > shrink_dcache_parent     prune_one_dentry
> > select_parent            dput     <<<< child is dead, locks are released,
> >                                       but parent is still referenced!!! >>>>
> > skip dentry->parent,
> > since it's d_count > 0
> >
> > message: BUSY inodes after umount...
> >                                  <<< parent is left on dentry_unused list,
> >                                      referencing freed super block >>>
> >
> > This patch is introducing dput_locked() which is doing all the dput work
> > except of freeing up the dentry's inode and memory itself. Therefore, when the
> > dcache_lock is given up, all the reference counts of the parents are correct.
> > prune_one_dentry() must also use the dput_locked version and free up the
> > inodes and the memory of the parents later. Otherwise we have an incorrect
> > reference count on the parents of the dentry to prune.
> >
> > ...
>
>
> > -void dput(struct dentry *dentry)
> > +static void dput_locked(struct dentry *dentry, struct list_head *list)
> > {
> > 	if (!dentry)
> > 		return;
> >
> > -repeat:
> > -	if (atomic_read(&dentry->d_count) == 1)
> > -		might_sleep();
> > -	if (!atomic_dec_and_lock(&dentry->d_count, &dcache_lock))
> > +	if (!atomic_dec_and_test(&dentry->d_count))
> > 		return;
> >
> > +
> >
> > ...
> >
> > +void dput(struct dentry *dentry)
> > +{
> > +	LIST_HEAD(free_list);
> > +
> > +	if (!dentry)
> > +		return;
> > +
> > +	if (atomic_add_unless(&dentry->d_count, -1, 1))
> > +		return;
> > +
> > +	spin_lock(&dcache_lock);
> > +	dput_locked(dentry, &free_list);
> > +	spin_unlock(&dcache_lock);
>
>
> This seems to be an open-coded copy of atomic_dec_and_lock()?

Yeah, this is what I also didn't like...
Why do it this way, when it's _really_ _possible_ to use old-good 
atomic_dec_and_test() keeping logic more clear?

Kirill

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