Hi,

Writing to /proc/sys/kernel/watchdog_thresh causes the following BUG in
at least v3.13-rc2-625-g06151db, v3.15 and v3.16-rc1. Kernel config is
attached.

It was originally found on bare metal, since then reproduced in QEMU in
init, and when directly executing it.

Regards,
Peter

[    0.676701] BUG: sleeping function called from invalid context at /tmp/linux/mm/slub.c:965
[    0.679396] in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: init
[    0.681204] 3 locks held by init/1:
[    0.682371]  #0:  (sb_writers#3){.+.+.+}, at: [<ffffffff8117b663>] vfs_write+0x143/0x180
[    0.685887]  #1:  (watchdog_proc_mutex){+.+.+.}, at: [<ffffffff810e02d3>] proc_dowatchdog+0x33/0x110
[    0.689631]  #2:  (cpu_hotplug.lock){.+.+.+}, at: [<ffffffff810589c2>] get_online_cpus+0x32/0x80
[    0.693117] Preemption disabled at:[<ffffffff810e0384>] proc_dowatchdog+0xe4/0x110
[    0.695753] 
[    0.696588] CPU: 0 PID: 1 Comm: init Not tainted 3.16.0-rc1-testing #34
[    0.698404] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[    0.704622]  ffff88003f250000 ffff88003f1ffd10 ffffffff81624c59 0000000000000000
[    0.707749]  ffff88003f1ffd30 ffffffff8108df1d 0000000000000010 ffffffff81c56360
[    0.711010]  ffff88003f1ffd78 ffffffff8116a5de ffffffff811159a5 ffffffff810a5f4a
[    0.714053] Call Trace:
[    0.715015]  [<ffffffff81624c59>] dump_stack+0x4e/0x7a
[    0.716619]  [<ffffffff8108df1d>] __might_sleep+0x11d/0x190
[    0.718232]  [<ffffffff8116a5de>] kmem_cache_alloc_trace+0x4e/0x1e0
[    0.720214]  [<ffffffff811159a5>] ? perf_event_alloc+0x55/0x440
[    0.721910]  [<ffffffff810a5f4a>] ? mark_held_locks+0x6a/0x90
[    0.723558]  [<ffffffff811159a5>] perf_event_alloc+0x55/0x440
[    0.725304]  [<ffffffff810e0050>] ? restart_watchdog_hrtimer+0x50/0x50
[    0.727279]  [<ffffffff81116d16>] perf_event_create_kernel_counter+0x26/0xe0
[    0.729269]  [<ffffffff810dfe05>] watchdog_nmi_enable+0x75/0x140
[    0.730965]  [<ffffffff810dffb3>] update_timers_all_cpus+0x53/0xa0
[    0.732953]  [<ffffffff810e0384>] proc_dowatchdog+0xe4/0x110
[    0.738408]  [<ffffffff811d8c93>] proc_sys_call_handler+0xb3/0xc0
[    0.740266]  [<ffffffff811d8cb4>] proc_sys_write+0x14/0x20
[    0.742086]  [<ffffffff8117b5cd>] vfs_write+0xad/0x180
[    0.743669]  [<ffffffff810a606d>] ? trace_hardirqs_on_caller+0xfd/0x1c0
[    0.745593]  [<ffffffff8117b799>] SyS_write+0x49/0xb0
[    0.747101]  [<ffffffff8162fe92>] system_call_fastpath+0x16/0x1b
[    0.749069] NMI watchdog: disabled (cpu0): hardware events not enabled