Re: [Linux-fbdev-devel] [2.6.29-rc2] fb_mmap: circular locking dependency on hibernation

From: Andrey Borzenkov
Date: Sun Feb 01 2009 - 02:16:10 EST

On 31 of January 2009 20:09:52 Andrea Righi wrote:
> On 2009-01-30 05:15, Andrey Borzenkov wrote:
> > On 29 of January 2009 12:10:11 Geert Uytterhoeven wrote:
> >> On Tue, 27 Jan 2009, Andrey Borzenkov wrote:
> >>> I do not know if this is new; I usually do not hibernate at all
> >>> and was testing another driver actually. I do not remember such
> >>> issues in 2.6.28 though. Dmesg follows; config attached.
> >>>
> >>> [11715.652498]
> >>> =======================================================
> >>> [11715.652520] [ INFO: possible circular locking dependency
> >>> detected ] [11715.652534] 2.6.29-rc2-1avb #11
> >>> [11715.652541]
> >>> -------------------------------------------------------
> >>> [11715.652550] s2disk/25519 is trying to acquire lock:
> >>> [11715.652562] (&fb_info->lock){--..}, at: [<c0223017>]
> >>> fb_mmap+0x97/0x170 [11715.652623]
> >>> [11715.652625] but task is already holding lock:
> >>> [11715.652632] (&mm->mmap_sem){----}, at: [<c010639e>]
> >>> sys_mmap2+0x8e/0xc0 [11715.652669]
> >>> [11715.652671] which lock already depends on the new lock.
> >>> [11715.652675]
> >>> [11715.652681]
> >>> [11715.652683] the existing dependency chain (in reverse order)
> >>> is: [11715.652691]
> >>> [11715.652693] -> #1 (&mm->mmap_sem){----}:
> >>> [11715.652707] [<c0144ecf>] __lock_acquire+0x129f/0x1930
> >>> [11715.652730] [<c01455bc>] lock_acquire+0x5c/0x80
> >>> [11715.652742] [<c0181e57>] might_fault+0x77/0xa0
> >>> [11715.652764] [<c0209325>] copy_from_user+0x35/0x130
> >>> [11715.652782] [<c0223ef5>] fb_ioctl+0x1d5/0x3f0
> >>> [11715.652797] [<c01a51d0>] vfs_ioctl+0x20/0x80
> >>> [11715.652832] [<c01a53e2>] do_vfs_ioctl+0x72/0x570
> >>> [11715.652845] [<c01a5919>] sys_ioctl+0x39/0x70
> >>> [11715.652857] [<c0103371>] sysenter_do_call+0x12/0x31
> >>> [11715.652870] [<ffffffff>] 0xffffffff
> >>> [11715.652973]
> >>> [11715.652975] -> #0 (&fb_info->lock){--..}:
> >>> [11715.652988] [<c014503c>] __lock_acquire+0x140c/0x1930
> >>> [11715.653000] [<c01455bc>] lock_acquire+0x5c/0x80
> >>> [11715.653011] [<c030761a>] mutex_lock_nested+0xba/0x2f0
> >>> [11715.653032] [<c0223017>] fb_mmap+0x97/0x170
> >>> [11715.653044] [<c0188996>] mmap_region+0x1d6/0x530
> >>> [11715.653067] [<c0188ec3>] do_mmap_pgoff+0x1d3/0x2f0
> >>> [11715.653079] [<c01063bd>] sys_mmap2+0xad/0xc0
> >>> [11715.653091] [<c0103371>] sysenter_do_call+0x12/0x31
> >>> [11715.653102] [<ffffffff>] 0xffffffff
> >>> [11715.653116]
> >>> [11715.653119] other info that might help us debug this:
> >>> [11715.653122]
> >>> [11715.653130] 1 lock held by s2disk/25519:
> >>> [11715.653137] #0: (&mm->mmap_sem){----}, at: [<c010639e>]
> >>> sys_mmap2+0x8e/0xc0
> >>> [11715.653156]
> >>> [11715.653158] stack backtrace:
> >>> [11715.653170] Pid: 25519, comm: s2disk Not tainted
> >>> 2.6.29-rc2-1avb #11 [11715.653179] Call Trace:
> >>> [11715.653190] [<c0306248>] ? printk+0x18/0x20
> >>> [11715.653202] [<c01437df>] print_circular_bug_tail+0xcf/0xe0
> >>> [11715.653213] [<c014503c>] __lock_acquire+0x140c/0x1930
> >>> [11715.653224] [<c014328b>] ? trace_hardirqs_on+0xb/0x10
> >>> [11715.653242] [<c0309125>] ? _spin_unlock_irqrestore+0x35/0x60
> >>> [11715.653254] [<c0140dc5>] ? lock_release_holdtime+0x35/0x210
> >>> [11715.653265] [<c01455bc>] lock_acquire+0x5c/0x80
> >>> [11715.653277] [<c0223017>] ? fb_mmap+0x97/0x170
> >>> [11715.653287] [<c030761a>] mutex_lock_nested+0xba/0x2f0
> >>> [11715.653298] [<c0223017>] ? fb_mmap+0x97/0x170
> >>> [11715.653309] [<c0223017>] ? fb_mmap+0x97/0x170
> >>> [11715.653335] [<c019514d>] ? kmem_cache_alloc+0xad/0x100
> >>> [11715.653347] [<c0223017>] fb_mmap+0x97/0x170
> >>> [11715.653358] [<c0188996>] mmap_region+0x1d6/0x530
> >>> [11715.653371] [<c0186a9d>] ?
> >>> arch_get_unmapped_area_topdown+0x10d/0x170 [11715.653383]
> >>> [<c0188ec3>] do_mmap_pgoff+0x1d3/0x2f0
> >>> [11715.653395] [<c01063bd>] sys_mmap2+0xad/0xc0
> >>> [11715.653406] [<c0103371>] sysenter_do_call+0x12/0x31
> >>
> >> Is this fixed by http://patchwork.kernel.org/patch/3259/?
> >
> > No. It just makes stack a bit more deep :)
> >
> > [ 408.267429]
> > ======================================================= [
> > 408.267448] [ INFO: possible circular locking dependency detected ]
> > [ 408.267462] 2.6.29-rc3-1avb #14
> > [ 408.267469]
> > ------------------------------------------------------- [
> > 408.267478] s2disk/5695 is trying to acquire lock:
> > [ 408.267487] (&fb_info->lock){--..}, at: [<c02235e7>]
> > fb_mmap+0x97/0x170
> > [ 408.267544]
> > [ 408.267546] but task is already holding lock:
> > [ 408.267553] (&mm->mmap_sem){----}, at: [<c010639e>]
> > sys_mmap2+0x8e/0xc0
> > [ 408.267591]
> > [ 408.267593] which lock already depends on the new lock.
> > [ 408.267596]
> > [ 408.267603]
> > [ 408.267604] the existing dependency chain (in reverse order) is:
> > [ 408.267612]
> > [ 408.267614] -> #3 (&mm->mmap_sem){----}:
> > [ 408.267628] [<c014520f>] __lock_acquire+0x129f/0x1930
> > [ 408.267650] [<c01458fc>] lock_acquire+0x5c/0x80
> > [ 408.267661] [<c0182117>] might_fault+0x77/0xa0
> > [ 408.267683] [<c0209916>] copy_to_user+0x36/0x120
> > [ 408.267699] [<c01a60f7>] filldir+0x97/0xe0
> > [ 408.267735] [<c01de339>] sysfs_readdir+0x129/0x220
> > [ 408.267751] [<c01a62a6>] vfs_readdir+0x86/0xa0
> > [ 408.267763] [<c01a63e8>] sys_getdents+0x68/0xc0
> > [ 408.267775] [<c010344a>] syscall_call+0x7/0xb
> > [ 408.267787] [<ffffffff>] 0xffffffff
> > [ 408.267862]
> > [ 408.267864] -> #2 (sysfs_mutex){--..}:
> > [ 408.267877] [<c014520f>] __lock_acquire+0x129f/0x1930
> > [ 408.267889] [<c01458fc>] lock_acquire+0x5c/0x80
> > [ 408.267899] [<c0307f3a>] mutex_lock_nested+0xba/0x2f0
> > [ 408.267921] [<c01de6cc>] sysfs_addrm_start+0x2c/0xc0
> > [ 408.267933] [<c01dec80>] create_dir+0x40/0x90
> > [ 408.267944] [<c01decfb>] sysfs_create_dir+0x2b/0x50
> > [ 408.267955] [<c02036bc>] kobject_add_internal+0xbc/0x1b0
> > [ 408.267984] [<c0203881>] kobject_add_varg+0x31/0x50
> > [ 408.267996] [<c02038fc>] kobject_add+0x2c/0x60
> > [ 408.268008] [<c0273448>] device_add+0xa8/0x550
> > [ 408.268041] [<c0273902>] device_register+0x12/0x20
> > [ 408.268053] [<c02739bb>] device_create_vargs+0xab/0xc0
> > [ 408.268065] [<c02739f8>] device_create+0x28/0x30
> > [ 408.268077] [<c026cc6d>] register_con_driver+0xed/0x130
> > [ 408.268090] [<c026e18b>] take_over_console+0x1b/0x50
> > [ 408.268102] [<c023052d>] fbcon_takeover+0x5d/0xb0
> > [ 408.268125] [<c02311a0>] fbcon_event_notify+0x820/0x900
> > [ 408.268137] [<c030c3b3>] notifier_call_chain+0x53/0xa0
> > [ 408.268161] [<c0138ab4>]
> > __blocking_notifier_call_chain+0x44/0x60
> > [ 408.268184] [<c0138aea>]
> > blocking_notifier_call_chain+0x1a/0x20
> > [ 408.268197] [<c0222f81>] fb_notifier_call_chain+0x11/0x20
> > [ 408.268209] [<c0223f08>] register_framebuffer+0x168/0x220
> > [ 408.268223] [<c0414ba2>] vesafb_probe+0x542/0x783
> > [ 408.268250] [<c02765af>] platform_drv_probe+0xf/0x20
> > [ 408.268269] [<c02757c7>] driver_probe_device+0x87/0x1b0
> > [ 408.268281] [<c0275988>] __device_attach+0x8/0x10
> > [ 408.268291] [<c0274e0b>] bus_for_each_drv+0x5b/0x80
> > [ 408.268308] [<c0275a36>] device_attach+0x76/0x80
> > [ 408.268319] [<c0274c27>] bus_attach_device+0x47/0x70
> > [ 408.268331] [<c02736c3>] device_add+0x323/0x550
> > [ 408.268342] [<c0276f85>] platform_device_add+0x175/0x1c0
> > [ 408.268358] [<c0414e7d>] vesafb_init+0x9a/0x1ec
> > [ 408.268370] [<c010111a>] do_one_initcall+0x2a/0x160
> > [ 408.268382] [<c04004d5>] kernel_init+0x83/0xd5
> > [ 408.268411] [<c0103a77>] kernel_thread_helper+0x7/0x10
> > [ 408.268423] [<ffffffff>] 0xffffffff
> > [ 408.268436]
> > [ 408.268438] -> #1 ((fb_notifier_list).rwsem){----}:
> > [ 408.268451] [<c014520f>] __lock_acquire+0x129f/0x1930
> > [ 408.268463] [<c01458fc>] lock_acquire+0x5c/0x80
> > [ 408.268474] [<c0308879>] down_read+0x49/0x90
> > [ 408.268484] [<c0138a9a>]
> > __blocking_notifier_call_chain+0x2a/0x60
> > [ 408.268497] [<c0138aea>]
> > blocking_notifier_call_chain+0x1a/0x20
> > [ 408.268509] [<c0222f81>] fb_notifier_call_chain+0x11/0x20
> > [ 408.268521] [<c022459e>] do_fb_ioctl+0x2ae/0x5d0
> > [ 408.268533] [<c02248dd>] fb_ioctl+0x1d/0x20
> > [ 408.268545] [<c01a54b0>] vfs_ioctl+0x20/0x80
> > [ 408.268558] [<c01a56c2>] do_vfs_ioctl+0x72/0x570
> > [ 408.268570] [<c01a5bf9>] sys_ioctl+0x39/0x70
> > [ 408.268581] [<c0103371>] sysenter_do_call+0x12/0x31
> > [ 408.268592] [<ffffffff>] 0xffffffff
> > [ 408.268623]
> > [ 408.268625] -> #0 (&fb_info->lock){--..}:
> > [ 408.268638] [<c014537c>] __lock_acquire+0x140c/0x1930
> > [ 408.268649] [<c01458fc>] lock_acquire+0x5c/0x80
> > [ 408.268661] [<c0307f3a>] mutex_lock_nested+0xba/0x2f0
> > [ 408.268672] [<c02235e7>] fb_mmap+0x97/0x170
> > [ 408.268684] [<c0188c76>] mmap_region+0x1d6/0x530
> > [ 408.268706] [<c01891a3>] do_mmap_pgoff+0x1d3/0x2f0
> > [ 408.268718] [<c01063bd>] sys_mmap2+0xad/0xc0
> > [ 408.268729] [<c0103371>] sysenter_do_call+0x12/0x31
> > [ 408.268740] [<ffffffff>] 0xffffffff
> > [ 408.268752]
> > [ 408.268754] other info that might help us debug this:
> > [ 408.268758]
> > [ 408.268767] 1 lock held by s2disk/5695:
> > [ 408.268773] #0: (&mm->mmap_sem){----}, at: [<c010639e>]
> > sys_mmap2+0x8e/0xc0
> > [ 408.268792]
> > [ 408.268794] stack backtrace:
> > [ 408.268805] Pid: 5695, comm: s2disk Not tainted 2.6.29-rc3-1avb
> > #14 [ 408.268814] Call Trace:
> > [ 408.268825] [<c0306b68>] ? printk+0x18/0x20
> > [ 408.268836] [<c0143b1f>] print_circular_bug_tail+0xcf/0xe0
> > [ 408.268847] [<c014537c>] __lock_acquire+0x140c/0x1930
> > [ 408.268862] [<c01487f0>] ? futex_wait+0x170/0x450
> > [ 408.268881] [<c011826c>] ? try_to_wake_up+0x10c/0x120
> > [ 408.268894] [<c0309a45>] ? _spin_unlock_irqrestore+0x35/0x60
> > [ 408.268906] [<c0141105>] ? lock_release_holdtime+0x35/0x210
> > [ 408.268917] [<c01458fc>] lock_acquire+0x5c/0x80
> > [ 408.268928] [<c02235e7>] ? fb_mmap+0x97/0x170
> > [ 408.268939] [<c0307f3a>] mutex_lock_nested+0xba/0x2f0
> > [ 408.268950] [<c02235e7>] ? fb_mmap+0x97/0x170
> > [ 408.268961] [<c02235e7>] ? fb_mmap+0x97/0x170
> > [ 408.268987] [<c019542d>] ? kmem_cache_alloc+0xad/0x100
> > [ 408.268999] [<c02235e7>] fb_mmap+0x97/0x170
> > [ 408.269010] [<c0188c76>] mmap_region+0x1d6/0x530
> > [ 408.269023] [<c0186d7d>] ?
> > arch_get_unmapped_area_topdown+0x10d/0x170
> > [ 408.269035] [<c01891a3>] do_mmap_pgoff+0x1d3/0x2f0
> > [ 408.269047] [<c01063bd>] sys_mmap2+0xad/0xc0
> > [ 408.269058] [<c0103371>] sysenter_do_call+0x12/0x31
>
> Anyway, looking at this trace in detail, it seems that in the fbcon
> notifier the FB_EVENT_SET_CONSOLE_MAP handler doesn't use fb_info at
> all.
>
> So, just for this particular case, we could unlock fb_info->lock
> before calling the routine that might acquire mm->mmap_sem, and avoid
> the circular locking dependency.
>
> Any chance to do a test with the following patch?
>

Step by step ... :)

With both patches applied:

[ 113.949848] =======================================================
[ 113.950054] [ INFO: possible circular locking dependency detected ]
[ 113.950054] 2.6.29-rc3-1avb #16
[ 113.950054] -------------------------------------------------------
[ 113.950054] s2disk/4168 is trying to acquire lock:
[ 113.950054] (&fb_info->lock){--..}, at: [<c0230c56>]
fbcon_event_notify+0x2d6/0x950
[ 113.950054]
[ 113.950054] but task is already holding lock:
[ 113.950054] ((fb_notifier_list).rwsem){----}, at: [<c0138a9a>]
__blocking_notifier_call_chain+0x2a/0x60
[ 113.950054]
[ 113.950054] which lock already depends on the new lock.
[ 113.950054]
[ 113.950054]
[ 113.950054] the existing dependency chain (in reverse order) is:
[ 113.950054]
[ 113.950054] -> #1 ((fb_notifier_list).rwsem){----}:
[ 113.950054] [<c014520f>] __lock_acquire+0x129f/0x1930
[ 113.950054] [<c01458fc>] lock_acquire+0x5c/0x80
[ 113.950054] [<c03088c9>] down_read+0x49/0x90
[ 113.950054] [<c0138a9a>] __blocking_notifier_call_chain+0x2a/0x60
[ 113.950054] [<c0138aea>] blocking_notifier_call_chain+0x1a/0x20
[ 113.950054] [<c0222f81>] fb_notifier_call_chain+0x11/0x20
[ 113.950054] [<c022459e>] do_fb_ioctl+0x2ae/0x5d0
[ 113.950054] [<c02248dd>] fb_ioctl+0x1d/0x20
[ 113.950054] [<c01a54b0>] vfs_ioctl+0x20/0x80
[ 113.950054] [<c01a56c2>] do_vfs_ioctl+0x72/0x570
[ 113.950054] [<c01a5bf9>] sys_ioctl+0x39/0x70
[ 113.950054] [<c0103371>] sysenter_do_call+0x12/0x31
[ 113.950054] [<ffffffff>] 0xffffffff
[ 113.950054]
[ 113.950054] -> #0 (&fb_info->lock){--..}:
[ 113.950054] [<c014537c>] __lock_acquire+0x140c/0x1930
[ 113.950054] [<c01458fc>] lock_acquire+0x5c/0x80
[ 113.950054] [<c0307f8a>] mutex_lock_nested+0xba/0x2f0
[ 113.950054] [<c0230c56>] fbcon_event_notify+0x2d6/0x950
[ 113.950054] [<c030c403>] notifier_call_chain+0x53/0xa0
[ 113.950054] [<c0138ab4>] __blocking_notifier_call_chain+0x44/0x60
[ 113.950054] [<c0138aea>] blocking_notifier_call_chain+0x1a/0x20
[ 113.950054] [<c0222f81>] fb_notifier_call_chain+0x11/0x20
[ 113.950054] [<c02246ca>] do_fb_ioctl+0x3da/0x5d0
[ 113.950054] [<c02248dd>] fb_ioctl+0x1d/0x20
[ 113.950054] [<c01a54b0>] vfs_ioctl+0x20/0x80
[ 113.950054] [<c01a56c2>] do_vfs_ioctl+0x72/0x570
[ 113.950054] [<c01a5bf9>] sys_ioctl+0x39/0x70
[ 113.950054] [<c0103371>] sysenter_do_call+0x12/0x31
[ 113.950054] [<ffffffff>] 0xffffffff
[ 113.950054]
[ 113.950054] other info that might help us debug this:
[ 113.950054]
[ 113.950054] 1 lock held by s2disk/4168:
[ 113.950054] #0: ((fb_notifier_list).rwsem){----}, at: [<c0138a9a>]
__blocking_notifier_call_chain+0x2a/0x60
[ 113.950054]
[ 113.950054] stack backtrace:
[ 113.950054] Pid: 4168, comm: s2disk Not tainted 2.6.29-rc3-1avb #16
[ 113.950054] Call Trace:
[ 113.950054] [<c0306bb8>] ? printk+0x18/0x20
[ 113.950054] [<c0143b1f>] print_circular_bug_tail+0xcf/0xe0
[ 113.950054] [<c014537c>] __lock_acquire+0x140c/0x1930
[ 113.950054] [<c0141105>] ? lock_release_holdtime+0x35/0x210
[ 113.950054] [<c0143992>] ? check_noncircular+0x92/0x150
[ 113.950054] [<c01463ce>] ? lock_release_non_nested+0x16e/0x2a0
[ 113.950054] [<c0138a9a>] ? __blocking_notifier_call_chain+0x2a/0x60
[ 113.950054] [<c01458fc>] lock_acquire+0x5c/0x80
[ 113.950054] [<c0230c56>] ? fbcon_event_notify+0x2d6/0x950
[ 113.950054] [<c0307f8a>] mutex_lock_nested+0xba/0x2f0
[ 113.950054] [<c0230c56>] ? fbcon_event_notify+0x2d6/0x950
[ 113.950054] [<c0230c56>] ? fbcon_event_notify+0x2d6/0x950
[ 113.950054] [<c0230c56>] fbcon_event_notify+0x2d6/0x950
[ 113.950054] [<c010b9ab>] ? save_stack_trace+0x2b/0x50
[ 113.950054] [<c014185c>] ? save_trace+0x3c/0xa0
[ 113.950054] [<c01418fd>] ? add_lock_to_list+0x3d/0xb0
[ 113.950054] [<c0141105>] ? lock_release_holdtime+0x35/0x210
[ 113.950054] [<c01462fd>] ? lock_release_non_nested+0x9d/0x2a0
[ 113.950054] [<c0138a9a>] ? __blocking_notifier_call_chain+0x2a/0x60
[ 113.950054] [<c030c403>] notifier_call_chain+0x53/0xa0
[ 113.950054] [<c03088e5>] ? down_read+0x65/0x90
[ 113.950054] [<c0138ab4>] __blocking_notifier_call_chain+0x44/0x60
[ 113.950054] [<c0138aea>] blocking_notifier_call_chain+0x1a/0x20
[ 113.950054] [<c0222f81>] fb_notifier_call_chain+0x11/0x20
[ 113.950054] [<c02246ca>] do_fb_ioctl+0x3da/0x5d0
[ 113.950054] [<c01820e6>] ? might_fault+0x46/0xa0
[ 113.950054] [<c01820e6>] ? might_fault+0x46/0xa0
[ 113.950054] [<c0182131>] ? might_fault+0x91/0xa0
[ 113.950054] [<c01820e6>] ? might_fault+0x46/0xa0
[ 113.950054] [<c0141105>] ? lock_release_holdtime+0x35/0x210
[ 113.950054] [<c01462fd>] ? lock_release_non_nested+0x9d/0x2a0
[ 113.950054] [<c01820e6>] ? might_fault+0x46/0xa0
[ 113.950054] [<c01820e6>] ? might_fault+0x46/0xa0
[ 113.950054] [<c0182131>] ? might_fault+0x91/0xa0
[ 113.950054] [<c01820e6>] ? might_fault+0x46/0xa0
[ 113.950054] [<c0209916>] ? copy_to_user+0x36/0x120
[ 113.950054] [<c02248c0>] ? fb_ioctl+0x0/0x20
[ 113.950054] [<c02248dd>] fb_ioctl+0x1d/0x20
[ 113.950054] [<c01a54b0>] vfs_ioctl+0x20/0x80
[ 113.950054] [<c01a56c2>] do_vfs_ioctl+0x72/0x570
[ 113.950054] [<c019c762>] ? sys_fstat64+0x22/0x30
[ 113.950054] [<c01a2180>] ? __link_path_walk+0x240/0xd40
[ 113.950054] [<c010339f>] ? sysenter_exit+0xf/0x16
[ 113.950054] [<c01a5bf9>] sys_ioctl+0x39/0x70
[ 113.950054] [<c0103371>] sysenter_do_call+0x12/0x31
[ 117.846986] Syncing filesystems ... done.

In case it helps - suspend to RAM does not exhibit this issue.

> Thanks,
> -Andrea
>
> ---
> fbcon: avoid circular locking dependency between fb_info->lock and
> mm->mmap_sem
>
> In fbcon notifier the handler for FB_EVENT_SET_CONSOLE_MAP doesn't
> need to hold fb_info->lock.
>
> Simply unlock it before calling set_con2fb_map(), that could try to
> acquire mm->mmap_sem to avoid a circular locking dependency with
> fb_mmap() (that acquires mm->mmap_sem -> fb_info-lock).
>
> Reported-by: Andrey Borzenkov <arvidjaar@xxxxxxx>
> Signed-off-by: Andrea Righi <righi.andrea@xxxxxxxxx>
> ---
> drivers/video/console/fbcon.c | 6 ++++++
> 1 files changed, 6 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/video/console/fbcon.c
> b/drivers/video/console/fbcon.c index 1657b96..81fc2f7 100644
> --- a/drivers/video/console/fbcon.c
> +++ b/drivers/video/console/fbcon.c
> @@ -3169,9 +3169,15 @@ static int fbcon_event_notify(struct
> notifier_block *self, ret = fbcon_fb_unregistered(info);
> break;
> case FB_EVENT_SET_CONSOLE_MAP:
> + /*
> + * XXX: info is not used here, we can unlock it to avoid
> + * circular locking with &current->mm->mmap_sem in fb_mmap().
> + */
> + unlock_fb_info(info);
> con2fb = event->data;
> ret = set_con2fb_map(con2fb->console - 1,
> con2fb->framebuffer, 1);
> + lock_fb_info(info);
> break;
> case FB_EVENT_GET_CONSOLE_MAP:
> con2fb = event->data;

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