linux-kernel - Re: [PATCH v4 1/2] SCSI: Asynchronous event notification infrastructure

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Date:	Mon, 29 Oct 2007 12:07:34 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	James Bottomley <James.Bottomley@...elEye.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Linux-SCSI <linux-scsi@...r.kernel.org>,
	akpm@...ux-foundation.org
Subject: Re: [PATCH v4 1/2] SCSI: Asynchronous event notification	infrastructure

James Bottomley wrote:
> This still doesn't solve the fundamental corruption problem:
> sdev->event_work has to contain the work entry until the workqueue has
> finished executing it (which is some unspecified time in the future).
> As soon as you drop the sdev->list_lock, the system thinks
> sdev->event_work is available for reuse.  If we fire another event
> before the work queue finished processing the prior event, the queue
> will be corrupted.

I think you're misunderstanding the workqueue code?  You can call 
schedule_work(&sdev->event_work) from anywhere, any time you like, as 
many times as you like.

> Also, I think Kristin's initial use of execute_in_user_context() was a
> good call .. if we already have a user context, there's no need to
> bother the workqueue ... some of these events will likely trigger from
> thread backed kernel daemons.

Quite agreed that sdev_evt_notify() might be called from kernel daemons, 
but in general this is a fire-and-forget API that is -likely- to be 
called from interrupt or completion context of many drivers, just like 
scsi_done or other completion APIs.  It is a fundamentally parallel 
interface.

If thread-backed kernel daemons want to use this, it is trivial for them 
to schedule work, then sync.

	Jeff

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