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

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Message-ID: <20090126173257.GE7591@atrey.karlin.mff.cuni.cz>
Date:	Mon, 26 Jan 2009 18:32:57 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Paul Clements <paul.clements@...eleye.com>
Cc:	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: nbd: add locking to nbd_ioctl

> Pavel Machek wrote:
> >>Pavel Machek wrote:
> >>>>Pavel Machek wrote:
> >>>>>On Fri 2009-01-16 10:24:06, Paul Clements wrote:
> >>>>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)
> >>>Well, unless the user is evil or confused? :-).
> >>Even in that case, you're just going to get EBUSY. Nothing bad will 
> >>happen. SET_SOCK checks for lo->file, so it cannot be called on an 
> >>active nbd device.
> >>
> >>
> >>>>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.
> >>>Ok, I'll take a look at the missing checks. I'd really like to make
> >>>this "stable" -- no amount of misuse should crash the kernel.
> >>Just to summarize, I don't think we need to hold tx_lock around the 
> >>entirety of nbd_ioctl. We do need one extra tx_lock around xmit_timeout 
> >>and we do need to check for lo->file and return EBUSY in all of the 
> >>SET_*SIZE* ioctls.
> >
> >I could do that but it would be a bit too complex, and still rely on
> >big kernel lock. Would you agree to patch that added tx_lock around
> >all of it, and moved ioctl to unlocked ioctl?
> 
> OK, I can buy the complexity argument. Your patch sounds fine to me.

Thanks for review!
								Pavel

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