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Date: Wed, 31 Jan 2024 23:02:43 +0100 (CET)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Tejun Heo <tj@kernel.org>
cc: torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, 
    dm-devel@lists.linux.dev, msnitzer@redhat.com, ignat@cloudflare.com, 
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    allen.lkml@gmail.com, kernel-team@meta.com, 
    Alasdair Kergon <agk@redhat.com>, Mike Snitzer <snitzer@kernel.org>
Subject: Re: [PATCH 8/8] dm-verity: Convert from tasklet to BH workqueue
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On Wed, 31 Jan 2024, Tejun Heo wrote:

> Hello,
> 
> On Wed, Jan 31, 2024 at 10:19:07PM +0100, Mikulas Patocka wrote:
> > > @@ -83,7 +83,7 @@ struct dm_verity_io {
> > >  	struct bvec_iter iter;
> > >  
> > >  	struct work_struct work;
> > > -	struct tasklet_struct tasklet;
> > > +	struct work_struct bh_work;
> > >  
> > >  	/*
> > >  	 * Three variably-size fields follow this struct:
> > 
> > Do we really need two separate work_structs here? They are never submitted 
> > concurrently, so I think that one would be enough. Or, am I missing 
> > something?
> 
> I don't know, so just did the dumb thing. If the caller always guarantees
> that the work items are never queued at the same time, reusing is fine.
> However, the followings might be useful to keep on mind:
> 
> - work_struct is pretty small - 4 pointers.
> 
> - INIT_WORK() on a queued work item isn't gonna be pretty.
> 
> - Flushing and no-concurrent-execution guarantee are broken on INIT_WORK().
>   e.g. If you queue_work(), INIT_WORK(), flush_work(), the flush isn't
>   actually going to wait for the work item to finish. Also, if you do
>   queue_work(), INIT_WORK(), queue_work(), the two queued work item
>   instances may end up running concurrently.
> 
> Muxing a single work item carries more risks of subtle bugs, but in some
> cases, the way it's used is clear (e.g. sequential chaining) and that's
> fine.

The code doesn't call INIT_WORK() on a queued work item and it doesn't 
flush the workqueue (it destroys it only in a situation when there are no 
work items running) so I think it's safe to use just one work_struct.

Mikulas