BUG: sleeping function called from invalid context (arm CONFIG_PREEMPT_RT_FULL )

[Till Straumann <strauman@slac.stanford.edu>]

I - very reproducibly - get this 'BUG' message

[ 6462.460032] Unhandled fault: external abort on non-linefetch (0x018) 
at 0xb6fdd000
[ 6462.460042] BUG: sleeping function called from invalid context at 
kernel/locking/rtmutex.c:905
[ 6462.460049] in_atomic(): 0, irqs_disabled(): 128, pid: 1488, name: ldfilt
[ 6462.460053] no locks held by ldfilt/1488.
[ 6462.460057] irq event stamp: 1790
[ 6462.460081] hardirqs last  enabled at (1789): [<c000ed10>] 
no_work_pending+0x8/0x2c
[ 6462.460096] hardirqs last disabled at (1790): [<c05bf834>] 
__dabt_usr+0x34/0x40
[ 6462.460116] softirqs last  enabled at (0): [<c0021594>] 
copy_process.part.50+0x498/0x170c
[ 6462.460124] softirqs last disabled at (0): [<  (null)>]   (null)
[ 6462.460135] CPU: 0 PID: 1488 Comm: ldfilt Tainted: G           O 
3.14.12-rt9-xilinx #25
[ 6462.460161] [<c0015f6c>] (unwind_backtrace) from [<c0012cc0>] 
(show_stack+0x20/0x24)
[ 6462.460182] [<c0012cc0>] (show_stack) from [<c05ba9ac>] 
(dump_stack+0x7c/0xcc)
[ 6462.460208] [<c05ba9ac>] (dump_stack) from [<c00574b8>] 
(__might_sleep+0x1a0/0x1d8)
[ 6462.460225] [<c00574b8>] (__might_sleep) from [<c05bea40>] 
(rt_spin_lock+0x30/0x64)
[ 6462.460240] [<c05bea40>] (rt_spin_lock) from [<c0036b44>] 
(force_sig_info+0x38/0xe8)
[ 6462.460254] [<c0036b44>] (force_sig_info) from [<c00130c0>] 
(arm_notify_die+0x50/0x60)
[ 6462.460266] [<c00130c0>] (arm_notify_die) from [<c000845c>] 
(do_DataAbort+0x94/0xa8)
[ 6462.460280] [<c000845c>] (do_DataAbort) from [<c05bf83c>] 
(__dabt_usr+0x3c/0x40)
[ 6462.460285] Exception stack(0xd2e65fb0 to 0xd2e65ff8)
[ 6462.460295] 5fa0:                                     0189d008 
00000001 00001000 b6fdd000
[ 6462.460308] 5fc0: 00011cf0 b6fbc078 0189d008 00009530 00000000 
be9b9ad0 ffffffff 00000000
[ 6462.460317] 5fe0: 00000000 be9b9a98 b6fa6c30 00008ad4 20000010 ffffffff
[ 6462.478073] Unhandled fault: external abort on non-linefetch (0x018) 
at 0xb6f2a000

on my CONFIG_PREEMPT_RT_FULL system:
#uname -a
Linux buildroot 3.14.12-rt9 #25 SMP PREEMPT RT Fri Nov 28 09:42:05 PST 
2014 armv7l GNU/Linux

when accessing a mmapped, non-existing device from user-space.

I'm not an ARM expert but I suspect that when the exception is taken
interrupts are disabled and probably not re-enabled by the exception
handler (irqs_disabled(): 128).

arm_notify_die() calls force_sig_info() which may block (under 
RT_PREEMPT_FULL).

In 'force_sig_info()' we find

/*
  * On some archs, PREEMPT_RT has to delay sending a signal from a trap
  * since it can not enable preemption, and the signal code's spin_locks
  * turn into mutexes. Instead, it must set TIF_NOTIFY_RESUME which will
  * send the signal on exit of the trap.
  */
#ifdef ARCH_RT_DELAYS_SIGNAL_SEND

and if this CPP symbol is defined there is a codepath that
delays signal delivery and never blocks.

Perhaps the arm support should use this facility?

Unfortunately I'm not familiar enough with this CPU arch to propose
a fix.

Best regards
- Till

PS: Please CC me on any replies since I'm not a lkml subscriber; thanks.