linux-kernel - Re: [Bug #15202] lockdep warning during elevator

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Date:	Sun, 7 Feb 2010 16:46:25 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>
Subject: Re: [Bug #15202] lockdep warning during elevator_switch

On Sun, 7 Feb 2010, Hugh Dickins wrote:

> > This message has been generated automatically as a part of a summary report
> > of recent regressions.
> > 
> > The following bug entry is on the current list of known regressions
> > from 2.6.32.  Please verify if it still should be listed and let the tracking team
> > know (either way).
> > 
> > 
> > Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=15202
> > Subject		: lockdep warning during elevator_switch
> > Submitter	: Hugh Dickins <hugh.dickins@...cali.co.uk>
> > Date		: 2010-01-31 23:55 (8 days old)
> > References	: http://marc.info/?l=linux-kernel&m=126498212613051&w=4
> 
> Yes, still so in 2.6.33-rc7.
> 

This type of lock dependency is always going to exist whenever a write to 
a sysfs file causes the kernel to remove its dirent.  In this case, 
writing to /sys/devices/block/xxx/queue/scheduler takes sd->dep_map and 
sd->parent->dep_map via sysfs_get_active_two() to pin both via 
sysfs_write_file().  The call to kobject_del() removes its directory 
from the hierarchy which takes sd->dep_map in sysfs_deactivate() because 
of 846f99749a.  Eric, do we need to preempt the rwsem from being acquired 
here when the dirent is already pinned because of a store to one of its 
files?
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