Re: [PATCH v4 1/1] dcache: Translating dentry into pathname without taking rename_lock

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Sep 09, 2013 at 10:45:38AM -0700, Linus Torvalds wrote:
> On Mon, Sep 9, 2013 at 10:29 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > I'm not sure I like mixing rcu_read_lock() into that - d_path() and friends
> > can do that themselves just fine (it needs to be taken when seq is even),
> > and e.g. d_walk() doesn't need it at all.  Other than that, I'm OK with
> > this variant.
> 
> Hmm.. I think you need the RCU read lock even when you get the write_seqlock().
> 
> Yes, getting the seqlock for write implies that you get a spinlock and
> in many normal circumstances that basically is equvalent to being
> rcu-locked, but afaik in some configurations that is *not* sufficient
> protection against an RCU grace period on another CPU. You need to do
> a real rcu_read_lock that increments that whole rcu_read_lock_nesting
> level, which a spinlock won't do.
> 
> And while the rename sequence lock protects against _renames_, it does
> not protect against just plain dentries getting free'd under memory
> pressure.

It protects the chain of ->d_parent, so they'd better not get freeds at
all...

> So I think the RCU-readlockness really needs to be independent of the
> sequence lock.

Actually, now that I've tried to convert d_walk() to those guys, I think
I like my proposal for the set of primitives better:

static inline bool seqretry_and_lock(seqlock_t *lock, unsigned *seq):
{
        if ((*seq & 1) || !read_seqretry(lock, *seq))
                return true;
        *seq |= 1;
        write_seqlock(lock);
        return false;
}

static inline void seqretry_done(seqlock_t *lock, unsigned seq)
{
        if (seq & 1)
                write_sequnlock(lock);
}

with the prepend_path() and friends becoming

	rcu_read_lock();
	seq = read_seqbegin(&rename_lock);
again:
	....
	if (!seqretry_and_lock(&rename_lock, seq))
		goto again;	/* now as writer */
	seqretry_done(&rename_lock, seq);
	rcu_read_unlock();

The thing is, d_walk() does essentially

	seq = read_seqbegin(&rename_lock);
again:
	....
	spin_lock(&d->d_lock);
	if (!seqretry_and_lock(&rename_lock, seq)) {
		spin_unlock(&d->d_lock);
		goto again;	/* now as writer */
	}
	/* now we are holding ->d_lock on it and we know
	 * that d has not gone stale until that point.
	 */	
	do stuff with d
	spin_unlock(&d->d_lock);
	seqretry_done(&rename_lock, seq);

OTOH, it's not impossible to handle with Waiman's primitives, just more
massage to do that...