perf: perf_fuzzer crashes on Pentium 4 systems

[Vince Weaver]

so moving this to its own thread.

There was a two-part question asked.
	1. Can the perf-fuzzer crash a Pentium 4 system
	2. Does anyone care anymore?

The answer to #1 turns out to be "yes"
I'm not sure about #2 (but it's telling my p4 test system hadn't been 
turned on in over 3 years).

In any case the perf_fuzzer can crash my p4 system within an hour or so.  
The debugging from this isn't great, I forget what the preferred debug 
things to enable in the kernel hacking menu are.

Here is one crash that just happened:

The instruction at RIP is unhelpfully
	./arch/x86/include/asm/processor.h:400
which is
	DECLARE_PER_CPU_FIRST(union irq_stack_union, irq_stack_union) __visible;

Though looking at the assembly it looks like
	p4_pmu_enable_event() is called with NULL as the paramater.

[ 1930.122902] BUG: unable to handle kernel NULL pointer dereference at 0000000000000158
[ 1930.130715] #PF error: [normal kernel read fault]
[ 1930.135402] PGD 0 P4D 0 
[ 1930.137928] Oops: 0000 [#1] SMP PTI
[ 1930.141405] CPU: 0 PID: 30179 Comm: perf_fuzzer Tainted: G        W         5.1.0-rc3+ #6
[ 1930.149555] Hardware name: LENOVO 88088NU/LENOVO, BIOS 2JKT37AUS 07/12/2007
[ 1930.156497] RIP: 0010:p4_pmu_enable_event+0x10/0x160
[ 1930.161443] Code: 89 f0 0f 30 31 c0 8b 15 e6 2e 0f 01 85 d2 7f 01 c3 89 c2 89 cf e9 70 65 3b 00 0f 1f 44 00 00 41 56 41 55 41 54 49 89 fc 55 53 <48> 8b 9f 58 01 00 00 48 89 dd 48 89 da 48 c1 ed 20 48 c1 ea 3f 89
[ 1930.180155] RSP: 0018:ffffc90001f57d50 EFLAGS: 00010017
[ 1930.185361] RAX: 0000000000000000 RBX: 000000000000000c RCX: 0000000000000360
[ 1930.192472] RDX: 0000000000000000 RSI: 0000000000000400 RDI: 0000000000000000
[ 1930.199582] RBP: ffff88803e40f620 R08: 00000000ffffffff R09: 0000000c00000000
[ 1930.206691] R10: 4801fe0000fc0000 R11: 8000000fce030200 R12: 0000000000000000
[ 1930.213802] R13: ffff888035c4a0c0 R14: ffff88803e429300 R15: 0000000000000402
[ 1930.220913] FS:  00007ff3b934a540(0000) GS:ffff88803e400000(0000) knlGS:0000000000000000
[ 1930.228976] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1930.234700] CR2: 0000000000000158 CR3: 000000003a72e000 CR4: 00000000000007f0
[ 1930.241811] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1930.248921] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
[ 1930.256030] Call Trace:
[ 1930.258472]  p4_pmu_enable_all+0x3c/0x50
[ 1930.262384]  __perf_event_task_sched_in+0x174/0x1a0
[ 1930.267247]  ? __switch_to_asm+0x34/0x70
[ 1930.271155]  ? __switch_to_asm+0x40/0x70
[ 1930.275064]  ? __switch_to_asm+0x34/0x70
[ 1930.278971]  ? __switch_to_asm+0x40/0x70
[ 1930.282882]  finish_task_switch+0x10a/0x290
[ 1930.287053]  __schedule+0x207/0x800
[ 1930.290530]  ? event_function_call+0x85/0x100
[ 1930.294873]  ? ctx_resched+0xc0/0xc0
[ 1930.298437]  preempt_schedule_common+0xa/0x20
[ 1930.302777]  _cond_resched+0x1d/0x30
[ 1930.306340]  mutex_lock+0xe/0x30
[ 1930.309558]  perf_event_ctx_lock_nested.isra.89+0x46/0x90
[ 1930.314939]  ? _perf_event_disable+0x40/0x40
[ 1930.319193]  perf_event_task_enable+0x3f/0xa0
[ 1930.323537]  __x64_sys_prctl+0x1b2/0x560
[ 1930.327448]  do_syscall_64+0x4f/0xf0
[ 1930.331011]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 1930.336045] RIP: 0033:0x7ff3b928240a