linux-kernel - Re: nbd: add locking to nbd

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Date:	Fri, 16 Jan 2009 11:28:10 -0500
From:	Paul Clements <paul.clements@...eleye.com>
To:	Pavel Machek <pavel@...e.cz>
CC:	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: nbd: add locking to nbd_ioctl

Pavel Machek wrote:
> On Fri 2009-01-16 10:24:06, Paul Clements wrote:
>> Pavel Machek wrote:
>>> The code was written with "oh big kernel lock, please protect me from
>>> all the evil" mentality: it does not locks its own data structures, it
>>> just hopes that big kernel lock somehow helps.
>>>
>>> It does not. (My fault).
>>>
>>> So this uses tx_lock to protect data structures from concurrent use
>>> between ioctl and worker threads.
>> What is the particular problem that this fixes? I thought we had already  
>> been careful to take tx_lock where necessary to protect data structures.  
>>   Perhaps there is something I missed?
> 
> for example lo->sock / lo->file are written to without holding any
> lock in current code. (lo->xmit_timeout has similar problem, and other
> fields, too).

lo->sock is only modified under tx_lock (except for SET_SOCK, where the 
device is being initialized, in which case it's impossible for any other 
thread to be accessing the device)

no one else uses lo->file except for the ioctls

I agree that if you really misuse the ioctls you could potentially get 
yourself in trouble with the xmit_timeout (the timer not being deleted 
or initialized properly if you hit the correct window). Taking tx_lock 
would prevent this.

As for other fields, I assume you're talking about blksize, et al. 
Taking tx_lock doesn't prevent you from screwing yourself if you modify 
those while the device is active. You'd need to disallow those ioctls 
when the device is active (check lo->file). Again, this is only going to 
happen if you really misuse the ioctls.

--
Paul
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