Re: [PATCH v3 19/19] fs: handle inode->i_version more efficiently

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2017-12-18 at 17:34 +0100, Jan Kara wrote:
> On Mon 18-12-17 10:11:56, Jeff Layton wrote:
> >  static inline bool
> >  inode_maybe_inc_iversion(struct inode *inode, bool force)
> >  {
> > -	atomic64_t *ivp = (atomic64_t *)&inode->i_version;
> > +	u64 cur, old, new;
> >  
> > -	atomic64_inc(ivp);
> > +	cur = (u64)atomic64_read(&inode->i_version);
> > +	for (;;) {
> > +		/* If flag is clear then we needn't do anything */
> > +		if (!force && !(cur & I_VERSION_QUERIED))
> > +			return false;
> 
> The fast path here misses any memory barrier. Thus it seems this query
> could be in theory reordered before any store that happened to modify the
> inode? Or maybe we could race and miss the fact that in fact this i_version
> has already been queried? But maybe there's some higher level locking that
> makes sure this is all a non-issue... But in that case it would deserve
> some comment I guess.
> 

There's no higher-level locking. Getting locking out of this codepath is
a good thing IMO. The larger question here is whether we really care
about ordering this with anything else.

The i_version, as implemented today, is not ordered with actual changes
to the inode. We only take the i_lock today when modifying it, not when
querying it. It's possible today that you could see the results of a
change and then do a fetch of the i_version that doesn't show an
increment vs. a previous change.

It'd be nice if this were atomic with the actual changes that it
represents, but I think that would be prohibitively expensive. That may
be something we need to address. I'm not sure we really want to do it as
part of this patchset though.

> > +
> > +		/* Since lowest bit is flag, add 2 to avoid it */
> > +		new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT;
> > +
> > +		old = atomic64_cmpxchg(&inode->i_version, cur, new);
> > +		if (likely(old == cur))
> > +			break;
> > +		cur = old;
> > +	}
> >  	return true;
> >  }
> >  
> 
> ...
> 
> >  static inline u64
> >  inode_query_iversion(struct inode *inode)
> >  {
> > -	return inode_peek_iversion(inode);
> > +	u64 cur, old, new;
> > +
> > +	cur = atomic64_read(&inode->i_version);
> > +	for (;;) {
> > +		/* If flag is already set, then no need to swap */
> > +		if (cur & I_VERSION_QUERIED)
> > +			break;
> > +
> > +		new = cur | I_VERSION_QUERIED;
> > +		old = atomic64_cmpxchg(&inode->i_version, cur, new);
> > +		if (old == cur)
> > +			break;
> > +		cur = old;
> > +	}
> 
> Why not just use atomic64_or() here?
> 

If the cmpxchg fails, then either:

1) it was incremented
2) someone flagged it QUERIED

If an increment happened then we don't need to flag it as QUERIED if
we're returning an older value. If we use atomic64_or, then we can't
tell if an increment happened so we'd end up potentially flagging it
more than necessary.

In principle, either outcome is technically OK and we don't have to loop
if the cmpxchg doesn't work. That said, if we think there might be a
later i_version available, then I think we probably want to try to query
it again so we can return as late a one as possible.
-- 
Jeff Layton <jlayton@kernel.org>