Hello Sebastian,
similar situation here with 3.18.11-rt7. The problem occurred once so far.
<1>[127209.359676] BUG: unable to handle kernel NULL pointer dereference at 0000001c
<1>[127209.359685] IP: [<c107ada9>] __try_to_take_rt_mutex.part.15+0x19/0x110
<4>[127209.359687] *pde = 00000000
<4>[127209.359689] Oops: 0000 [#1] PREEMPT SMP
<4>[127209.359716] Modules linked in: tun krfb serport max6639 rtc_isl12058 ramoops reed_solomon kmmi_cp300 led_class kbatt_leg i2c_keba_leg at25 m25p80 spi_nor ktick_cp300 kspimaster ksram kspi2 ecm cp300 ecmpci powerfail i2c_i801 video backlight processor adt7475 hwmon_vid emc1403 regmap_i2c coretemp loop fuse autofs4 uvesafb cfbfillrect cfbimgblt cn cfbcopyarea sg sd_mod ata_generic usb_storage ata_piix ehci_pci ehci_hcd e1000e ptp pps_core thermal thermal_sys hwmon
<0>[127209.359721] CPU: 0 PID: 2254 Comm: varpvprf Not tainted 3.18.0-none-keba-rt-686 #1 KEBA 3.18.11-1~~Test3+~d8
<0>[127209.359722] Hardware name: /BM67/BS67/TM67/TS67, BIOS BH67R98 17/12/2012
<0>[127209.359724] task: f5879880 ti: f504c000 task.ti: f504c000
<4>[127209.359727] EIP: 0060:[<c107ada9>] EFLAGS: 00010286 CPU: 0
<4>[127209.359730] EIP is at __try_to_take_rt_mutex.part.15+0x19/0x110
<4>[127209.359732] EAX: f5170504 EBX: f5170504 ECX: f504dda0 EDX: 00000000
<4>[127209.359733] ESI: f5879880 EDI: f5879d2c EBP: f504dd84 ESP: f504dd74
<4>[127209.359735] DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
<4>[127209.359737] CR0: 80050033 CR2: 0000001c CR3: 328cc000 CR4: 000407d0
<0>[127209.359737] Stack:
<4>[127209.359742] d84b24c0 f5170504 d84b24c0 f5879d2c f504dde0 c14ad99c 00000001 f5879880
<4>[127209.359746] f504dfec f5879880 f5879d2c 00000001 00000000 00000000 f504ddac 00000000
<4>[127209.359750] 00000000 f5879880 f5170504 11111101 11111111 00000000 11111111 00000078
<0>[127209.359751] Call Trace:
<4>[127209.359759] [<c14ad99c>] rt_spin_lock_slowlock+0x1ac/0x2f0
<4>[127209.359765] [<c14aee9d>] rt_spin_lock+0xd/0x10
<4>[127209.359769] [<c104f5d8>] __lock_task_sighand+0x38/0x70
<4>[127209.359774] [<c118ea82>] do_task_stat+0xf2/0xd30
<4>[127209.359778] [<c14ada84>] ? rt_spin_lock_slowlock+0x294/0x2f0
<4>[127209.359783] [<c1156785>] ? mntput_no_expire+0x25/0x150
<4>[127209.359788] [<c10649e5>] ? preempt_count_sub+0x45/0x50
<4>[127209.359792] [<c1064f00>] ? migrate_enable+0x80/0x180
<4>[127209.359797] [<c12b6607>] ? __percpu_counter_add+0xa7/0xf0
<4>[127209.359801] [<c14aeb85>] ? _raw_spin_lock+0x15/0x60
<4>[127209.359805] [<c14adcb0>] ? rt_mutex_slowlock+0x1d0/0x310
<4>[127209.359809] [<c10649e5>] ? preempt_count_sub+0x45/0x50
<4>[127209.359812] [<c14adcb0>] ? rt_mutex_slowlock+0x1d0/0x310
<4>[127209.359817] [<c118fdbf>] proc_tid_stat+0x1f/0x30
<4>[127209.359820] [<c118b7e2>] proc_single_show+0x42/0x80
<4>[127209.359825] [<c115aab2>] seq_read+0x82/0x380
<4>[127209.359829] [<c115aa30>] ? seq_lseek+0x1c0/0x1c0
<4>[127209.359834] [<c1138714>] vfs_read+0x74/0x140
<4>[127209.359838] [<c14aef9d>] ? _mutex_lock+0xd/0x10
<4>[127209.359842] [<c1138d36>] SyS_read+0x46/0x90
<4>[127209.359847] [<c14af59c>] sysenter_do_call+0x12/0x12
<0>[127209.359874] Code: 76 00 0f 0b 8d b4 26 00 00 00 00 8d bc 27 00 00 00 00 55 89 e5 57 56 53 83 ec 04 66 66 66 66 90 85 c9 89 c3 89 d6 74 22 8b 50 08 <3b> 42 1c 0f 85 ce 00 00 00 31 c0 39 d1 0f 84 cc 00 00 00 83 c4
<0>[127209.359879] EIP: [<c107ada9>] __try_to_take_rt_mutex.part.15+0x19/0x110 SS:ESP 0068:f504dd74
<4>[127209.359880] CR2: 000000000000001c
<4>[127209.396217] ---[ end trace 0000000000000002 ]---
regards, Gerhard Engleder
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