Re: [PATCH] btrfs: hold block_group refcount during async discard

[Boris Burkov <boris@bur.io>]

On Fri, Jan 13, 2023 at 02:26:25PM +0100, David Sterba wrote:
> On Thu, Jan 12, 2023 at 04:05:11PM -0800, Boris Burkov wrote:
> > Async discard does not acquire the block group reference count while it
> > holds a reference on the discard list. This is generally OK, as the
> > paths which destroy block groups tend to try to synchronize on
> > cancelling async discard work. However, relying on cancelling work
> > requires careful analysis to be sure it is safe from races with
> > unpinning scheduling more work.
> > 
> > While I am unable to find a race with unpinning in the current code for
> > either the unused bgs or relocation paths, I believe we have one in an
> > older version of auto relocation in a Meta internal build. This suggests
> > that this is in fact an error prone model, and could be fragile to
> > future changes to these bg deletion paths.
> 
> Which version is that? I'll add stable tag anyway but for a cross
> reference from a real deployment. Thanks.

I misunderstood the state of our branch. The code we were running has
Johannes's original patches for the reclaim worker and an internal only
patch that removes the pinned check. So I don't think there is any
commit in your tree which lacks the pinned check.