linux-kernel - Re: [PATCH 4/4] disk-protect: Add a generic block layer queue freezing facility

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Message-ID: <87k5k1klby.fsf@denkblock.local>
Date:	Tue, 18 Mar 2008 00:00:17 +0100
From:	Elias Oltmanns <eo@...ensachen.de>
To:	linux-ide@...r.kernel.org
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH 4/4] disk-protect: Add a generic block layer queue freezing facility

Elias Oltmanns <eo@...ensachen.de> wrote:
> Pavel Machek <pavel@....cz> wrote:
>> On Fri 2008-03-07 19:26:41, Elias Oltmanns wrote:
>>> This patch provides a generic way to freeze the request queue of a block
>>> device temporarily. This functionality is exposed to user space via sysfs.
>>> 
>>> Signed-off-by: Elias Oltmanns <eo@...ensachen.de>
[...]
>>> +/*
>>> + * When reading the 'protect' attribute, we return seconds remaining
>>> + * before the unfreeze timeout expires.
>>> + */
>>> +static ssize_t queue_protect_show(struct request_queue *q, char *page)
>>> +{
>>> +	unsigned int seconds = 0;
>>> +
>>> +	if (blk_queue_stopped(q) && timer_pending(&q->unfreeze_timer))
>>> +		/*
>>> +		 * Adding 1 in order to guarantee nonzero value until timer
>>> +		 * has actually expired.
>>> +		 */
>>> +		seconds = jiffies_to_msecs(q->unfreeze_timer.expires
>>> +					   - jiffies) / 1000 + 1;
>>
>> Is it okay to read expires without locking? 
>
> No, I have been a bit careless with regard to locking in this patch. See
> the revised version below.

Come to think of it, the lock really shouldn't be released between
modding / checking the timer and issuing the respective command. This
makes the whole thing a bit messy because allocating a request the
standard way is done before acquiring the spin_lock. Since we don't know
at that time whether a command is actually going to be enqueued, this
looks rather suboptimal to me. A better idea anyone?

Regards,

Elias


View attachment "04-block-freeze-queue.patch" of type "text/x-patch" (6439 bytes)