On Thu, Jul 09, 2020 at 08:55:26AM +0800, Gao Xiang wrote:
> On Thu, Jul 09, 2020 at 09:33:11AM +1000, Dave Chinner wrote:
> > On Tue, Jul 07, 2020 at 09:57:41PM +0800, Gao Xiang wrote:
> > > Currently, we use AGI buffer lock to protect in-memory linked list for
> > > unlinked inodes but since it's not necessary to modify AGI unless the
> > > head of the unlinked list is modified. So let's removing the AGI buffer
> > > modification dependency if possible, including 1) adding another per-AG
> > > dedicated lock to protect the whole list and 2) inserting unlinked
> > > inodes from tail.
> > >
> > > For 2), the tail of bucket 0 is now recorded in perag for xfs_iunlink()
> > > to use. xfs_iunlink_remove() still support old multiple short bucket
> > > lists for recovery code.
> >
> > I would split this into two separate patches. One to move to a perag
> > based locking strategy, another to change from head to tail
> > addition as they are largely independent algorithmic changes.
>
> Yes, that is much better from the perspective of spilting patches and
> I thought that before. It seems that is like 2 steps but the proposed
> target solution is as a whole (in other words, 2 steps are code-related)
> and I'm not sure how large these code is sharable or can be inherited
> but rather than introduce some code in patch 2 and then remove immediately
> and turn into a new code in patch 3). I'm not sure how large logic could
> be sharable between these 2 dependent steps so I didn't do that.
>
> I will spilt patches in the next RFC version to make a try.
Thanks!
[snip]
> > i.e. this:
> >
> > xfs_iunlink()
> > {
> >
> > get locks
> > do list insert
> > drop locks
> > }
> >
> > Is better for understanding, maintenance and future modification
> > than:
> >
> > xfs_iunlink()
> > {
> >
> > get perag
> > lock perag
> > look at tail of list
> > if (empty) {
> > unlock perag
> > read/lock AGI
> > lock perag
> > look at tail of list
> > if (empty)
> > do head insert
> > goto out
> > }
> > do tail insert
> > out:
> > update inode/pag tails
> > unlock
> > drop perag
> > }
> >
> > It's trivial for a reader to understand what the first version of
> > xfs_iunlink() is going to do without needing to understand the
> > intraccies of the locking strategies. However, it takes time and
> > effort to undestand exactly waht the second one is doing because
> > it's not clear where lock ends and list modifications start, nor
> > what the locking rules are for the different modifications that are
> > being made. Essentially, it goes back to the complex
> > locking-intertwined-with-modification-algorithm problem the current
> > TOT code has.
> >
> > I'd much prefer to see something like this:
> >
> > /*
> > * Inode allocation in the O_TMPFILE path defines the AGI/unlinked
> > * list lock order as being AGI->perag unlinked list lock. We are
> > * inverting it here as the fast path tail addition does not need to
> > * modify the AGI at all. Hence we only need the AGI lock if the
> > * tail is empty, but if we fail to get it without blocking then we
> > * need to fall back to the slower, correct lock order.
> > */
> > xfs_iunlink_insert_lock()
> > {
> > get perag;
> > lock_perag();
> > if (!tail empty)
> > return;
> > if (trylock AGI)
> > return;
>
> (adding some notes here, this patch doesn't use try lock here
> finally but unlock perag and take AGI and relock and recheck tail_empty....
> since the tail non-empty is rare...)
*nod*
My point was largely that this sort of thing is really obvious and
easy to optimise once the locking is cleanly separated. Adding a
trylock rather than drop/relock is another patch for the series :P
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
