linux-kernel - Re: [BUG] Circular locking dependency

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Date:	Wed, 12 Feb 2014 19:29:01 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	dri-devel@...ts.freedesktop.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG] Circular locking dependency - DRM/CMA/MM/hotplug/...

On Wed, Feb 12, 2014 at 04:33:17PM +0000, Russell King - ARM Linux wrote:
> On Wed, Feb 12, 2014 at 04:40:50PM +0100, Marek Szyprowski wrote:
> >> -> #3 (console_lock){+.+.+.}:
> >>         [<c0066f04>] __lock_acquire+0x151c/0x1ca0
> >>         [<c0067c28>] lock_acquire+0xa0/0x130
> >>         [<c006edcc>] console_lock+0x60/0x74
> >>         [<c006f7b8>] console_cpu_notify+0x28/0x34
> >>         [<c004904c>] notifier_call_chain+0x4c/0x8c
> >>         [<c004916c>] __raw_notifier_call_chain+0x1c/0x24
> >>         [<c0024124>] __cpu_notify+0x34/0x50
> >>         [<c002424c>] cpu_notify_nofail+0x18/0x24
> >>         [<c068e168>] _cpu_down+0x100/0x244
> >>         [<c068e2dc>] cpu_down+0x30/0x44
> >>         [<c036ef8c>] cpu_subsys_offline+0x14/0x18
> >>         [<c036af28>] device_offline+0x94/0xbc
> >>         [<c036b030>] online_store+0x4c/0x74
> >>         [<c0368d3c>] dev_attr_store+0x20/0x2c
> >>         [<c016b2e0>] sysfs_kf_write+0x54/0x58
> >>         [<c016eaa4>] kernfs_fop_write+0xc4/0x160
> >>         [<c0105a54>] vfs_write+0xbc/0x184
> >>         [<c0105dfc>] SyS_write+0x48/0x70
> >>         [<c000e6e0>] ret_fast_syscall+0x0/0x48
> 
> cpu_down() takes cpu_hotplug.lock, so here we have:
> 
> 	cpu_hotplug.lock
> 	console_lock

The patche I've linked in my other mail will break the chain here, so
should solve this. And apparently with cpu hotplug we can hit this, too.
And having banged my head against the console_lock wall I think doing a
trylock here is generally the sanest option.

So imo we can just blame console_lock, not need to either beat up v4l,
drm, cma or anyone else really ;-)

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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