linux-kernel - Re: [RFC PATCH 01/11] workqueue: Add a decrement-after-return and wake if 0 facility

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Date:   Tue, 5 Sep 2017 06:29:51 -0700
From:   Tejun Heo <tj@...nel.org>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 01/11] workqueue: Add a decrement-after-return and
 wake if 0 facility

Hello, David.

On Fri, Sep 01, 2017 at 04:40:53PM +0100, David Howells wrote:
> Add a facility to the workqueue subsystem whereby an atomic_t can be
> registered by a work function such that the work function dispatcher will
> decrement the atomic after the work function has returned and then call
> wake_up_atomic() on it if it reached 0.
> 
> This is analogous to complete_and_exit() for kernel threads and is used to
> avoid a race between notifying that a work item is about to finish and the
> .text segment from a module being discarded.
> 
> The way this is used is that the work function calls:
> 
> 	dec_after_work(atomic_t *counter);
> 
> to register the counter and then process_one_work() calls it, potentially
> wakes it and clears the registration.
> 
> The reason I've used an atomic_t rather than a completion is that (1) it
> takes up less space and (2) it can monitor multiple objects.

Given how work items are used, I think this is too inviting to abuses
where people build complex event chains through these counters and
those chains would be completely opaque.  If the goal is protecting
.text of a work item, can't we just do that?  Can you please describe
your use case in more detail?  Why can't it be done via the usual
"flush from exit"?

Thanks.

-- 
tejun