blkio + cfq was crashing even when two sequential readers were put in two
separate cgroups (group_isolation=0).

The reason being that cfqq can migrate across groups based on its being
sync-noidle or not, it can happen that at request insertion time, cfqq
belonged to one cfqg and at request dispatch time, it belonged to root
group. In this case request stats per cgroup can go wrong and it also runs
into BUG_ON().

This patch implements rq stashing away a cfq group pointer and not relying
on cfqq->cfqg pointer alone for rq stat accounting.

[   65.163523] ------------[ cut here ]------------
[   65.164301] kernel BUG at block/blk-cgroup.c:117!
[   65.164301] invalid opcode: 0000 [#1] SMP
[   65.164301] last sysfs file: /sys/devices/pci0000:00/0000:00:05.0/0000:60:00.1/host9/rport-9:0-0/target9:0:0/9:0:0:2/block/sde/stat
[   65.164301] CPU 1
[   65.164301] Modules linked in: dm_round_robin dm_multipath qla2xxx scsi_transport_fc dm_zero dm_mirror dm_region_hash dm_log dm_mod [last unloaded: scsi_wait_scan]
[   65.164301]
[   65.164301] Pid: 4505, comm: fio Not tainted 2.6.34-rc4-blk-for-35 #34 0A98h/HP xw8600 Workstation
[   65.164301] RIP: 0010:[<ffffffff8121924f>]  [<ffffffff8121924f>] blkiocg_update_io_remove_stats+0x5b/0xaf
[   65.164301] RSP: 0018:ffff8800ba5a79e8  EFLAGS: 00010046
[   65.164301] RAX: 0000000000000096 RBX: ffff8800bb268d60 RCX: 0000000000000000
[   65.164301] RDX: ffff8800bb268eb8 RSI: 0000000000000000 RDI: ffff8800bb268e00
[   65.164301] RBP: ffff8800ba5a7a08 R08: 0000000000000064 R09: 0000000000000001
[   65.164301] R10: 0000000000079640 R11: ffff8800a0bd5bf0 R12: ffff8800bab4af01
[   65.164301] R13: ffff8800bab4af00 R14: ffff8800bb1d8928 R15: 0000000000000000
[   65.164301] FS:  00007f18f75056f0(0000) GS:ffff880001e40000(0000) knlGS:0000000000000000
[   65.164301] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   65.164301] CR2: 000000000040e7f0 CR3: 00000000ba52b000 CR4: 00000000000006e0
[   65.164301] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   65.164301] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[   65.164301] Process fio (pid: 4505, threadinfo ffff8800ba5a6000, task ffff8800ba45ae80)
[   65.164301] Stack:
[   65.164301]  ffff8800ba5a7a08 ffff8800ba722540 ffff8800bab4af68 ffff8800bab4af68
[   65.164301] <0> ffff8800ba5a7a38 ffffffff8121d814 ffff8800ba722540 ffff8800bab4af68
[   65.164301] <0> ffff8800ba722540 ffff8800a08f6800 ffff8800ba5a7a68 ffffffff8121d8ca
[   65.164301] Call Trace:
[   65.164301]  [<ffffffff8121d814>] cfq_remove_request+0xe4/0x116
[   65.164301]  [<ffffffff8121d8ca>] cfq_dispatch_insert+0x84/0xe1
[   65.164301]  [<ffffffff8121e833>] cfq_dispatch_requests+0x767/0x8e8
[   65.164301]  [<ffffffff8120e524>] ? submit_bio+0xc3/0xcc
[   65.164301]  [<ffffffff810ad657>] ? sync_page_killable+0x0/0x35
[   65.164301]  [<ffffffff8120ea8d>] blk_peek_request+0x191/0x1a7
[   65.164301]  [<ffffffffa000109c>] ? dm_get_live_table+0x44/0x4f [dm_mod]
[   65.164301]  [<ffffffffa0002799>] dm_request_fn+0x38/0x14c [dm_mod]
[   65.164301]  [<ffffffff810ad657>] ? sync_page_killable+0x0/0x35
[   65.164301]  [<ffffffff8120f600>] __generic_unplug_device+0x32/0x37
[   65.164301]  [<ffffffff8120f8a0>] generic_unplug_device+0x2e/0x3c
[   65.164301]  [<ffffffffa00011a6>] dm_unplug_all+0x42/0x5b [dm_mod]
[   65.164301]  [<ffffffff8120b063>] blk_unplug+0x29/0x2d
[   65.164301]  [<ffffffff8120b079>] blk_backing_dev_unplug+0x12/0x14
[   65.164301]  [<ffffffff81108a82>] block_sync_page+0x35/0x39
[   65.164301]  [<ffffffff810ad64e>] sync_page+0x41/0x4a
[   65.164301]  [<ffffffff810ad665>] sync_page_killable+0xe/0x35
[   65.164301]  [<ffffffff81589027>] __wait_on_bit_lock+0x46/0x8f
[   65.164301]  [<ffffffff810ad52d>] __lock_page_killable+0x66/0x6d
[   65.164301]  [<ffffffff81055fd4>] ? wake_bit_function+0x0/0x33
[   65.164301]  [<ffffffff810ad560>] lock_page_killable+0x2c/0x2e
[   65.164301]  [<ffffffff810aebfd>] generic_file_aio_read+0x361/0x4f0
[   65.164301]  [<ffffffff810e906c>] do_sync_read+0xcb/0x108
[   65.164301]  [<ffffffff811e32a3>] ? security_file_permission+0x16/0x18
[   65.164301]  [<ffffffff810e96d3>] vfs_read+0xab/0x108
[   65.164301]  [<ffffffff810e97f0>] sys_read+0x4a/0x6e
[   65.164301]  [<ffffffff81002b5b>] system_call_fastpath+0x16/0x1b
[   65.164301] Code: 00 74 1c 48 8b 8b 60 01 00 00 48 85 c9 75 04 0f 0b eb fe 48 ff c9 48 89 8b 60 01 00 00 eb 1a 48 8b 8b 58 01 00 00 48 85 c9 75 04 <0f> 0b eb fe 48 ff c9 48 89 8b 58 01 00 00 45 84 e4 74 16 48 8b
[   65.164301] RIP  [<ffffffff8121924f>] blkiocg_update_io_remove_stats+0x5b/0xaf
[   65.164301]  RSP <ffff8800ba5a79e8>
[   65.164301] ---[ end trace 1b2b828753032e68 ]---
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>