Subject: Re: [PATCH 2/2] scsi: don't use execute_in_process_context()
From: James Bottomley <James.Bottomley@suse.de>
To: Tejun Heo <tj@kernel.org>
Cc: Linux SCSI List <linux-scsi@vger.kernel.org>,
        FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>,
        lkml <linux-kernel@vger.kernel.org>
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Date: Tue, 14 Dec 2010 22:04:05 -0500
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On Tue, 2010-12-14 at 15:33 +0100, Tejun Heo wrote:
> Hello,
> 
> On 12/14/2010 03:26 PM, James Bottomley wrote:
> > Depends what you're doing about the flush problem.  The synchronisation
> > is inherent in the use (we're holding a reference to the module within
> > the executed code).  The flush is to try to speed things up so the user
> > doesn't get annoyed during rmmod.  We don't need a sync, just an
> > accelerator.
> 
> Hmmm, I'm confused.  How does it drop the reference then?

Um, the same way it does in your new code: inside the executed function.

>   Something
> outside of the callback should wait for its completion and drop the
> reference as otherwise nothing can guarantee that the modules doesn't
> go away between the reference drop and the actual completion of the
> callback.

Well, if that's an actual problem, your patch doesn't solve it.  In both
cases the work structure is part of the object that will be released.
The way it should happen is that workqueues dequeue the work (so now no
refs) and execute the callback with the data, so the callback is OK to
free the work structure.  As long as it still does that, there's no
problem in either case.

> >> Compelling reason for it to exist.  Why not just use work when you
> >> need execution context and the caller might or might not have one?
> > 
> > Because it's completely lame to have user context and not use it.
> 
> It may be lame but I think it's better than having an optimization
> interface which is incomplete and, more importantly, unnecessary.

But you're thinking of it as a workqueue issue ... it isn't, it's an API
which says "just make sure I have user context".  The workqueue is just
the implementation detail.

> >> But, really, let's just remove it.  At this point, we either need to
> >> fortify the interface or remove it and given the current usage, I
> >> think we're better off with the latter.
> > 
> > I really don't think the open coding is a good idea.  It's complex and
> > error prone; exactly the type of thing that should be in an API.
> 
> Yeah, just schedule work like everyone else.

As I said: the required open coding then becomes error prone.

James


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