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Date: Thu, 05 Jun 2008 10:45:01 +0300
From: Andriy Gapon <avg@icyb.net.ua>
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To: Kostik Belousov <kostikbel@gmail.com>
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Cc: freebsd-fs@freebsd.org, freebsd-stable@freebsd.org,
	freebsd-geom@freebsd.org
Subject: Re: mystery: lock up after fs dump
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on 04/06/2008 19:10 Kostik Belousov said the following:
> SU are irrelevant to the problem I am thinking of.
> 
> vfs_write_suspend() returns 0 when the filesystem being suspended is already
> in suspend state. vfs_write_resume() clears the suspend state.
> 
> vfs_write_suspend/vfs_write_resume are used both by snapshot code and
> the gjournal. If two users of these interfaces interleave, then you could
> get:
> 
> 	thread1				thread2
> 
> 	vfs_write_suspend()
> 				<- fs is suspended there
> 					vfs_write_suspend() <- returns 0
> 	vfs_write_resume()
> 				<- fs is no more suspended
> 					thread2 is burned in flame
> 
> Snapshots are protected against this because they are created through
> the mount(2). The mount(2) locks the covered vnode and thus serializes
> snapshot creation (I think there are further serialization points that
> prevent simultaneous snapshotting of the same fs).
> 
> There is nothing I can see that protects snapshots/gjournal interaction.

Looks like something to be quite concerned about.
Thank you for the analysis.

-- 
Andriy Gapon