linux-kernel - Re: 2.6.21-mm1

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Date:	Sun, 6 May 2007 14:22:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Satyam Sharma" <satyam.sharma@...il.com>
Cc:	"Simon Arlott" <simon@...e.lp0.eu>, linux-kernel@...r.kernel.org,
	linux-dvb@...uxtv.org, Kay Sievers <kay.sievers@...y.org>,
	Trent Piepho <xyzzy@...akeasy.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: 2.6.21-mm1

On Mon, 7 May 2007 02:34:26 +0530 "Satyam Sharma" <satyam.sharma@...il.com> wrote:

> On 5/7/07, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > On Sun, 06 May 2007 15:59:53 +0100 Simon Arlott <simon@...e.lp0.eu> wrote:
> >
> > > On 05/05/07 09:49, Andrew Morton wrote:
> > > > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.21/2.6.21-mm1/
> > >
> > > I'm currently in the middle of a bisect over the last week of commits to linus'
> > > tree, but I got the following with -mm1 that isn't showing up in the latest
> > > 2.6.21-git:
> > >
> > > [   15.644778] BUG: sleeping function called from invalid context at kernel/mutex.c:86
> > > [   15.644873] in_atomic():1, irqs_disabled():1
> > > [   15.644882] 2 locks held by modprobe/2765:
> > > [   15.644889]  #0:  (devlist_lock){--..}, at: [<b052dc7f>] mutex_lock+0x1f/0x30
> > > [   15.645058]  #1:  (modlist_lock){....}, at: [<b0147b1d>] __symbol_get+0x1d/0x90
> > > [   15.645207] irq event stamp: 11772
> > > [   15.645271] hardirqs last  enabled at (11771): [<b016d793>] slab_free+0xc3/0x200
> > > [   15.645289] hardirqs last disabled at (11772): [<b052f476>] _spin_lock_irqsave+0x16/0x60
> > > [   15.645364] softirqs last  enabled at (11216): [<b0125286>] __do_softirq+0x96/0xb0
> > > [   15.645439] softirqs last disabled at (11211): [<b01069f2>] do_softirq+0x82/0x100
> > > [   15.645517]  [<b0104f0a>] show_trace_log_lvl+0x1a/0x30
> > > [   15.645586]  [<b0104f32>] show_trace+0x12/0x20
> > > [   15.645654]  [<b0105045>] dump_stack+0x15/0x20
> > > [   15.645665]  [<b011d6ad>] __might_sleep+0xcd/0xf0
> > > [   15.645737]  [<b052dc78>] mutex_lock+0x18/0x30
> > > [   15.645806]  [<b01b1b7c>] sysfs_create_link+0x6c/0x130
> > > [   15.645823]  [<b0146dba>] use_module+0x11a/0x170
> > > [   15.645892]  [<b0147b6c>] __symbol_get+0x6c/0x90
> > > [   15.645904]  [<f086ac2c>] dvb_init+0x98c/0xd70 [saa7134_dvb]
> > > [   15.645984]  [<b036861d>] mpeg_ops_attach+0x3d/0x50
> > > [   15.646058]  [<b0369109>] saa7134_ts_register+0x29/0x70
> > > [   15.646070]  [<f086f010>] dvb_register+0x10/0x12 [saa7134_dvb]
> > > [   15.646141]  [<b01491b7>] sys_init_module+0xf7/0x150
> > > [   15.646153]  [<b0104172>] sysenter_past_esp+0x5f/0x99
> > > [   15.646221]  =======================
> >
> > Looks like a locking error in the DVB code.
> 
> Actually, kernel/module.c:__symbol_get() does
> spinlock_irq_save(&modlist_lock) and then calls use_module().
> 
> But when CONFIG_MODULE_UNLOAD=y, use_module() wants to
> sysfs_create_link() (I wonder why) which will clearly blow up.

OK, thanks.

What appears to have hapened is that Trent's not-yet-in-mainline
add-ability-to-keep-track-of-callers-of-symbol_getput.patch assumes that
use_module() is atomic, but Kay's now-in-mainline
gregkh-driver-sys-modules-holders.patch concurrently assumed that
use_module() is not atomic.

But use_module() already had a GFP_ATOMIC allocation in it, which tends to
indicate that sticking a sysfs_create_link() into use_module() wasn't an
appropriate thing to do.

gargh.  Could we please have some statement regarding use_module()'s
caller-provided environment and locking expectations?


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