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Sender: Tejun Heo <htejun@gmail.com>
Date: Mon, 5 Feb 2024 13:58:29 -1000
From: Tejun Heo <tj@kernel.org>
To: Kent Overstreet <kent.overstreet@linux.dev>,
	Brian Foster <bfoster@redhat.com>
Cc: linux-bcachefs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] bcachefs: Use alloc_ordered_workqueue() to create ordered
 workqueues
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BACKGROUND
==========

When multiple work items are queued to a workqueue, their execution order
doesn't match the queueing order. They may get executed in any order and
simultaneously. When fully serialized execution - one by one in the queueing
order - is needed, an ordered workqueue should be used which can be created
with alloc_ordered_workqueue().

However, alloc_ordered_workqueue() was a later addition. Before it, an
ordered workqueue could be obtained by creating an UNBOUND workqueue with
@max_active==1. This originally was an implementation side-effect which was
broken by 4c16bd327c74 ("workqueue: restore WQ_UNBOUND/max_active==1 to be
ordered"). Because there were users that depended on the ordered execution,
5c0338c68706 ("workqueue: restore WQ_UNBOUND/max_active==1 to be ordered")
made workqueue allocation path to implicitly promote UNBOUND workqueues w/
@max_active==1 to ordered workqueues.

While this has worked okay, overloading the UNBOUND allocation interface
this way creates other issues. It's difficult to tell whether a given
workqueue actually needs to be ordered and users that legitimately want a
min concurrency level wq unexpectedly gets an ordered one instead. With
planned UNBOUND workqueue updates to improve execution locality and more
prevalence of chiplet designs which can benefit from such improvements, this
isn't a state we wanna be in forever.

This patch series audits all callsites that create an UNBOUND workqueue w/
@max_active==1 and converts them to alloc_ordered_workqueue() as necessary.

WHAT TO LOOK FOR
================

The conversions are from

  alloc_workqueue(WQ_UNBOUND | flags, 1, args..)

to 

  alloc_ordered_workqueue(flags, args...)

which don't cause any functional changes. If you know that fully ordered
execution is not ncessary, please let me know. I'll drop the conversion and
instead add a comment noting the fact to reduce confusion while conversion
is in progress.

If you aren't fully sure, it's completely fine to let the conversion
through. The behavior will stay exactly the same and we can always
reconsider later.

As there are follow-up workqueue core changes, I'd really appreciate if the
patch can be routed through the workqueue tree w/ your acks. Thanks.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Brian Foster <bfoster@redhat.com>
---
 fs/bcachefs/btree_update_interior.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/bcachefs/btree_update_interior.c
+++ b/fs/bcachefs/btree_update_interior.c
@@ -2484,7 +2484,7 @@ void bch2_fs_btree_interior_update_init_
 int bch2_fs_btree_interior_update_init(struct bch_fs *c)
 {
 	c->btree_interior_update_worker =
-		alloc_workqueue("btree_update", WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
+		alloc_ordered_workqueue("btree_update", WQ_MEM_RECLAIM);
 	if (!c->btree_interior_update_worker)
 		return -BCH_ERR_ENOMEM_btree_interior_update_worker_init;