perf_counter: Fix /0 bug in swcounters

perf_counter: Fix /0 bug in swcounters

We have a race in the swcounter stuff where we can start
counting a counter that has never been enabled, this leads to a
/0 situation.

The below avoids the /0 but doesn't close the race, this would
need a new counter state.

The race is due to perf_swcounter_is_counting() which cannot
discern between disabled due to scheduled out, and disabled for
any other reason.

Such a crash has been seen by Ingo:

[  967.092372] divide error: 0000 [#1] SMP
[  967.096499] last sysfs file: /sys/devices/system/cpu/cpu15/cache/index2/shared_cpu_map
[  967.104846] CPU 5
[  967.106965] Modules linked in:
[  967.110169] Pid: 3351, comm: hackbench Not tainted 2.6.31-rc8-tip-01158-gd940a54-dirty #1568 X8DTN
[  967.119456] RIP: 0010:[<ffffffff810c0aba>]  [<ffffffff810c0aba>] perf_swcounter_ctx_event+0x127/0x1af
[  967.129137] RSP: 0018:ffff8801a95abd70  EFLAGS: 00010046
[  967.134699] RAX: 0000000000000002 RBX: ffff8801bd645c00 RCX: 0000000000000002
[  967.142162] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8801bd645d40
[  967.149584] RBP: ffff8801a95abdb0 R08: 0000000000000001 R09: ffff8801a95abe00
[  967.157042] R10: 0000000000000037 R11: ffff8801aa1245f8 R12: ffff8801a95abe00
[  967.164481] R13: ffff8801a95abe00 R14: ffff8801aa1c0e78 R15: 0000000000000001
[  967.171953] FS:  0000000000000000(0000) GS:ffffc90000a00000(0063) knlGS:00000000f7f486c0
[  967.180406] CS:  0010 DS: 002b ES: 002b CR0: 000000008005003b
[  967.186374] CR2: 000000004822c0ac CR3: 00000001b19a2000 CR4: 00000000000006e0
[  967.193770] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  967.201224] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  967.208692] Process hackbench (pid: 3351, threadinfo ffff8801a95aa000, task ffff8801a96b0000)
[  967.217607] Stack:
[  967.219711]  0000000000000000 0000000000000037 0000000200000001 ffffc90000a1107c
[  967.227296] <0> ffff8801a95abe00 0000000000000001 0000000000000001 0000000000000037
[  967.235333] <0> ffff8801a95abdf0 ffffffff810c0c20 0000000200a14f30 ffff8801a95abe40
[  967.243532] Call Trace:
[  967.246103]  [<ffffffff810c0c20>] do_perf_swcounter_event+0xde/0xec
[  967.252635]  [<ffffffff810c0ca7>] perf_tpcounter_event+0x79/0x7b
[  967.258957]  [<ffffffff81037f73>] ftrace_profile_sched_switch+0xc0/0xcb
[  967.265791]  [<ffffffff8155f22d>] schedule+0x429/0x4c4
[  967.271156]  [<ffffffff8100c01e>] int_careful+0xd/0x14

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1251472247.17617.74.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>