linux-kernel - Re: [PATCH 06/11] uml ubd driver: ubd_io

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Message-ID: <20061030082643.GM4563@kernel.dk>
Date:	Mon, 30 Oct 2006 09:26:48 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Blaisorblade <blaisorblade@...oo.it>
Cc:	akpm@...l.org, jdike@...toit.com,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/11] uml ubd driver: ubd_io_lock usage fixup

On Sun, Oct 29 2006, Blaisorblade wrote:
> Add some comments about requirements for ubd_io_lock and expand its use.
> 
> When an irq signals that the "controller" (i.e. another thread on the host,
> which does the actual requests and is the only one blocked on I/O on the
>  host) has done some work, we call again the request function ourselves
> (do_ubd_request).
> 
> We now do that with ubd_io_lock held - that's useful to protect against
> concurrent calls to elv_next_request and so on.

Not only useful, required, as I think I complained about a year or more
ago :-)

> XXX: Maybe we shouldn't call at all the request function. Input needed on
>  this. Are we supposed to plug and unplug the queue? That code "indirectly"
>  does that by setting a flag, called do_ubd, which makes the request function
>  return (it's a residual of 2.4 block layer interface).

Sometimes you need to. I'd probably just remove the do_ubd check and
always recall the request function when handling completions, it's
easier and safe.

-- 
Jens Axboe

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