Re: [PATCH] fb: avoid possible deadlock caused by fb_set_suspend

From: Florian Tobias Schandinat
Date: Wed Jul 20 2011 - 14:16:30 EST

Hi Paul,

what are your plans for the patch below? Do you queue it for 3.1?
The only feedback so far was that one driver (sh_mobile_hdmi) would require more work with a patch for that proposed. Without any objections to the patch I think we should take it as fixing the subsystem is more important than causing potential issues with a single driver.


Florian Tobias Schandinat

On 06/17/2011 07:02 PM, Florian Tobias Schandinat wrote:
From: Herton Ronaldo Krzesinski<herton@xxxxxxxxxxxxxxx>

A lock ordering issue can cause deadlocks: in framebuffer/console code,
all needed struct fb_info locks are taken before acquire_console_sem(),
in places which need to take console semaphore.

But fb_set_suspend is always called with console semaphore held, and
inside it we call lock_fb_info which gets the fb_info lock, inverse
locking order of what the rest of the code does. This causes a real
deadlock issue, when we write to state fb sysfs attribute (which calls
fb_set_suspend) while a framebuffer is being unregistered by
remove_conflicting_framebuffers, as can be shown by following show
blocked state trace on a test program which loads i915 and runs another
forked processes writing to state attribute:

Test process with semaphore held and trying to get fb_info lock:
fb-test2 D 0000000000000000 0 237 228 0x00000000
ffff8800774f3d68 0000000000000082 00000000000135c0 00000000000135c0
ffff880000000000 ffff8800774f3fd8 ffff8800774f3fd8 ffff880076ee4530
00000000000135c0 ffff8800774f3fd8 ffff8800774f2000 00000000000135c0
Call Trace:
[<ffffffff8141287a>] __mutex_lock_slowpath+0x11a/0x1e0
[<ffffffff814142f2>] ? _raw_spin_lock_irq+0x22/0x40
[<ffffffff814123d3>] mutex_lock+0x23/0x50
[<ffffffff8125dfc5>] lock_fb_info+0x25/0x60
[<ffffffff8125e3f0>] fb_set_suspend+0x20/0x80
[<ffffffff81263e2f>] store_fbstate+0x4f/0x70
[<ffffffff812e7f70>] dev_attr_store+0x20/0x30
[<ffffffff811c46b4>] sysfs_write_file+0xd4/0x160
[<ffffffff81155a26>] vfs_write+0xc6/0x190
[<ffffffff81155d51>] sys_write+0x51/0x90
[<ffffffff8100c012>] system_call_fastpath+0x16/0x1b
modprobe process stalled because has the fb_info lock (got inside
unregister_framebuffer) but waiting for the semaphore held by the
test process which is waiting to get the fb_info lock:
modprobe D 0000000000000000 0 230 218 0x00000000
ffff880077a4d618 0000000000000082 0000000000000001 0000000000000001
ffff880000000000 ffff880077a4dfd8 ffff880077a4dfd8 ffff8800775a2e20
00000000000135c0 ffff880077a4dfd8 ffff880077a4c000 00000000000135c0
Call Trace:
[<ffffffff81411fe5>] schedule_timeout+0x215/0x310
[<ffffffff81058051>] ? get_parent_ip+0x11/0x50
[<ffffffff814130dd>] __down+0x6d/0xb0
[<ffffffff81089f71>] down+0x41/0x50
[<ffffffff810629ac>] acquire_console_sem+0x2c/0x50
[<ffffffff812ca53d>] unbind_con_driver+0xad/0x2d0
[<ffffffff8126f5f7>] fbcon_event_notify+0x457/0x890
[<ffffffff814144ff>] ? _raw_spin_unlock_irqrestore+0x1f/0x50
[<ffffffff81058051>] ? get_parent_ip+0x11/0x50
[<ffffffff8141836d>] notifier_call_chain+0x4d/0x70
[<ffffffff8108a3b8>] __blocking_notifier_call_chain+0x58/0x80
[<ffffffff8108a3f6>] blocking_notifier_call_chain+0x16/0x20
[<ffffffff8125dabb>] fb_notifier_call_chain+0x1b/0x20
[<ffffffff8125e6ac>] unregister_framebuffer+0x7c/0x130
[<ffffffff8125e8b3>] remove_conflicting_framebuffers+0x153/0x180
[<ffffffff8125eef3>] register_framebuffer+0x93/0x2c0
[<ffffffffa0331112>] drm_fb_helper_single_fb_probe+0x252/0x2f0 [drm_kms_helper]
[<ffffffffa03314a3>] drm_fb_helper_initial_config+0x2f3/0x6d0 [drm_kms_helper]
[<ffffffffa03318dd>] ? drm_fb_helper_single_add_all_connectors+0x5d/0x1c0 [drm_kms_helper]
[<ffffffffa037b588>] intel_fbdev_init+0xa8/0x160 [i915]
[<ffffffffa0343d74>] i915_driver_load+0x854/0x12b0 [i915]
[<ffffffffa02f0e7e>] drm_get_pci_dev+0x19e/0x360 [drm]
[<ffffffff8141821d>] ? sub_preempt_count+0x9d/0xd0
[<ffffffffa0386f91>] i915_pci_probe+0x15/0x17 [i915]
[<ffffffff8124481f>] local_pci_probe+0x5f/0xd0
[<ffffffff81244f89>] pci_device_probe+0x119/0x120
[<ffffffff812eccaa>] ? driver_sysfs_add+0x7a/0xb0
[<ffffffff812ed003>] driver_probe_device+0xa3/0x290
[<ffffffff812ed1f0>] ? __driver_attach+0x0/0xb0
[<ffffffff812ed29b>] __driver_attach+0xab/0xb0
[<ffffffff812ed1f0>] ? __driver_attach+0x0/0xb0
[<ffffffff812ebd3e>] bus_for_each_dev+0x5e/0x90
[<ffffffff812ecc2e>] driver_attach+0x1e/0x20
[<ffffffff812ec6f2>] bus_add_driver+0xe2/0x320
[<ffffffffa03aa000>] ? i915_init+0x0/0x96 [i915]
[<ffffffff812ed536>] driver_register+0x76/0x140
[<ffffffffa03aa000>] ? i915_init+0x0/0x96 [i915]
[<ffffffff81245216>] __pci_register_driver+0x56/0xd0
[<ffffffffa02f1264>] drm_pci_init+0xe4/0xf0 [drm]
[<ffffffffa03aa000>] ? i915_init+0x0/0x96 [i915]
[<ffffffffa02e84a8>] drm_init+0x58/0x70 [drm]
[<ffffffffa03aa094>] i915_init+0x94/0x96 [i915]
[<ffffffff81002194>] do_one_initcall+0x44/0x190
[<ffffffff810a066b>] sys_init_module+0xcb/0x210
[<ffffffff8100c012>] system_call_fastpath+0x16/0x1b

fb-test2 which reproduces above is available on bug #26232.
To solve this issue, avoid calling lock_fb_info inside fb_set_suspend,
and move it out to where needed (callers of fb_set_suspend must call
lock_fb_info before if needed). So far, the only place which needs to
call lock_fb_info is store_fbstate, all other places which calls
fb_set_suspend are suspend/resume hooks that should not need the lock as
they should be run only when processes are already frozen in

Signed-off-by: Herton Ronaldo Krzesinski<herton@xxxxxxxxxxxxxxx>
Signed-off-by: Florian Tobias Schandinat<FlorianSchandinat@xxxxxx>
Cc: stable@xxxxxxxxxx
drivers/video/fbmem.c | 3 ---
drivers/video/fbsysfs.c | 3 +++
2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
index 5aac00e..ad93629 100644
--- a/drivers/video/fbmem.c
+++ b/drivers/video/fbmem.c
@@ -1738,8 +1738,6 @@ void fb_set_suspend(struct fb_info *info, int state)
struct fb_event event;

- if (!lock_fb_info(info))
- return; = info;
if (state) {
@@ -1748,7 +1746,6 @@ void fb_set_suspend(struct fb_info *info, int state)
- unlock_fb_info(info);

diff --git a/drivers/video/fbsysfs.c b/drivers/video/fbsysfs.c
index 04251ce..67afa9c 100644
--- a/drivers/video/fbsysfs.c
+++ b/drivers/video/fbsysfs.c
@@ -399,9 +399,12 @@ static ssize_t store_fbstate(struct device *device,

state = simple_strtoul(buf,&last, 0);

+ if (!lock_fb_info(fb_info))
+ return -ENODEV;
fb_set_suspend(fb_info, (int)state);
+ unlock_fb_info(fb_info);

return count;

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