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Date:   Thu, 2 Mar 2023 06:01:56 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
Cc:     Qu Wenruo <quwenruo.btrfs@gmx.com>, David Sterba <dsterba@suse.cz>,
        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v7 04/13] btrfs: add support for inserting raid stripe
 extents
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On Thu, Mar 02, 2023 at 11:58:13AM +0000, Johannes Thumshirn wrote:
> > Thus it can cause deadlock if the workqueue has one max_active, and the 
> > running one is finish_ordered_fn(), which then can be waiting for the 
> > RST work.
> > 
> > But the RST work can only be executed if the endio_workers has finished 
> > its current work, thus leading to a deadlock.
> 
> How about adding a new workqueue for RST updates? That should mitigate
> the deadlock.

The amount of weird workqueues in the btrfs end I/O path is worrysome.
What I plan to do, and might be ready to submit about next week is a
series to actually offload the I/O completion to a workqueue on a
per (original) btrfs_bio basis.  This means that RST updates,
ordered_extent processing, compressed write handling etc can all
run from the same end I/O worker.  As a bonus we remove all irqsave
locking from btrfs.