主要有四种，加入backtrace的方法：
1. WARN_ON(cond)//比如 WARN_ON(!host->claimed);
条件满足时，输出如下log：
WARNING: at drivers/mmc/core/core.c:240 mmc_wait_for_cmd+0x2c/0x74() //by warn_slowpath_common
Modules linked in: bcm4330//by print_modules
[<8006df6c>] (unwind_backtrace+0x0/0xf0) from [<8008f7c8>] (warn_slowpath_common+0x4c/0x64) //by dump_stack
[<8008f7c8>] (warn_slowpath_common+0x4c/0x64) from [<8008f7f8>] (warn_slowpath_null+0x18/0x1c)
[<8008f7f8>] (warn_slowpath_null+0x18/0x1c) from [<802fdc0c>] (mmc_wait_for_cmd+0x2c/0x74)
[<802fdc0c>] (mmc_wait_for_cmd+0x2c/0x74) from [<802fffec>] (mmc_go_idle+0x60/0xac)
[<802fffec>] (mmc_go_idle+0x60/0xac) from [<80301288>] (sdio_reset_comm+0x2c/0x15c)
[<80301288>] (sdio_reset_comm+0x2c/0x15c) from [<7f01c410>] (sdioh_start+0x2c/0x10c [bcm4330])
[<7f01c410>] (sdioh_start+0x2c/0x10c [bcm4330]) from [<7f00c334>] (wl_control_wl_start+0x5c/0xa8 [bcm4330])
[<7f00c334>] (wl_control_wl_start+0x5c/0xa8 [bcm4330]) from [<7f00c38c>] (wl_iw_control_wl_on+0xc/0x34 [bcm4330])
[<7f00c38c>] (wl_iw_control_wl_on+0xc/0x34 [bcm4330]) from [<7f00cd14>] (wl_iw_set_priv+0xb4/0xb50 [bcm4330])
[<7f00cd14>] (wl_iw_set_priv+0xb4/0xb50 [bcm4330]) from [<8042e614>] (ioctl_standard_call+0x64/0x388)
[<8042e614>] (ioctl_standard_call+0x64/0x388) from [<8042e9e8>] (wext_handle_ioctl+0xb0/0x1f4)
[<8042e9e8>] (wext_handle_ioctl+0xb0/0x1f4) from [<80358604>] (dev_ioctl+0x754/0x784)
[<80358604>] (dev_ioctl+0x754/0x784) from [<8010088c>] (vfs_ioctl+0x2c/0xac)
[<8010088c>] (vfs_ioctl+0x2c/0xac) from [<80100f40>] (do_vfs_ioctl+0x540/0x5a0)
[<80100f40>] (do_vfs_ioctl+0x540/0x5a0) from [<80100fec>] (sys_ioctl+0x4c/0x6c)
[<80100fec>] (sys_ioctl+0x4c/0x6c) from [<80069960>] (ret_fast_syscall+0x0/0x30)
---[ end trace 1b75b31a2719ed1e ]--- //by print_oops_end_marker
2. BUG_ON(cond);//比如 BUG_ON(in_atomic());
3. 系统的OOPS，可能无法打印到kernel.log中，而是只能显示在串口中，比如：
[ 302.253861] Unable to handle kernel paging request at virtual address a0000013
[ 302.286587] pgd = cf5c4000
[ 302.296897] [a0000013] *pgd=00000000
[ 302.309751] Internal error: Oops: 805 [#1] PREEMPT
[ 302.314550] last sysfs file: /sys/devices/system/cpu/cpu0/cpufreq/stats/time_in_state
[ 302.322382] Modules linked in: [last unloaded: bcm4330]
[ 302.327634] CPU: 0 Not tainted (2.6.35.3+ #246)
[ 302.332516] PC is at 0x7f0870b4
[ 302.335666] LR is at del_timer+0x20/0xc4
[ 302.339592] pc : [<7f0870b4>] lr : [<8009ad60>] psr: 60000013
[ 302.339597] sp : cffe5e90 ip : 00000000 fp : 00000000
[ 302.351082] r10: c3061588 r9 : 00000000 r8 : 00000680
[ 302.356308] r7 : cf38772c r6 : cffe5ecf r5 : 00000000 r4 : 00000000
[ 302.362837] r3 : 000000c1 r2 : cffe5e60 r1 : a0000013 r0 : 00000680
[ 302.369368] Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment kernel
[ 302.376679] Control: 10c5387d Table: bf4f0019 DAC: 00000017
[ 302.382428] Process dhd_sysioc (pid: 2720, stack limit = 0xcffe42e8)
[ 302.388784] Stack: (0xcffe5e90 to 0xcffe6000)
[ 302.393144] 5e80: c3061400 00000000 cffdc000 8008c4fc
[ 302.401327] 5ea0: cf38772c cf38772c 00000000 c3061400 cf3870b0 00000000 cfe14000 0000001f
[ 302.409511] 5ec0: cfe14038 7f09dd50 c3061400 cfe14038 0000001f 00000000 cf3870b0 cfe14000
[ 302.417693] 5ee0: cfe14000 cff3e2a0 0000000f 7f0a144c ffffffff 00000000 00000000 00000062
[ 302.425876] 5f00: cfe14000 cfe14038 00000000 00000000 80854e30 809952e0 cf387800 cf3870b0
[ 302.434060] 5f20: cff3e2a0 cffe5fb0 00000000 cffe5fb0 c3061d18 cf387000 c3061c00 7f08d53c
[ 302.442242] 5f40: 0000000f 00000000 cffe5fb0 00000000 cf387000 cf3870b0 cffe4000 7f089414
[ 302.450425] 5f60: 0000000f 00000000 cff3e2af cff3e2a0 0000000f cf3877f0 cf3877d0 cf387890
[ 302.458607] 5f80: cf3878aa cf3877e0 cf387000 c3061c00 00000000 00000000 cffe5fac 8008c548
[ 302.466790] 5fa0: 00000000 00000000 00000000 800699ec 00000107 cff3e2a0 0000000f 00000001
[ 302.474972] 5fc0: 00000000 00000000 00000000 00000000 00000000 cf3877d0 7f0891b0 8006a3c0
[ 302.483155] 5fe0: 00000013 00000000 00000000 00000000 00000000 8006a3c0 71737011 71737411
[ 302.491355] [<8009ad60>] (del_timer+0x20/0xc4) from [<8008c4fc>] (default_wake_function+0x0/0x14)
[ 302.500236] Code: 00019970 0005571c 000199a8 0005441c (000199b4)
[ 302.506352] imx_ssi_irq mxc_ssi SISR 801121 SIER 180100 fifo_errs=1
[ 302.538716] enter mx_wifi_powerup
[ 302.639406]
[ 302.639416] end of mxc_mmc_force_detect
[ 302.644820] esdhc_cd_callback..cd_status=0 inserted
[ 302.694776] ---[ end trace 7ff6b3e3d944f8e7 ]---
[ 302.701656] Kernel panic - not syncing: Fatal exception
[ 302.706955] [<8006df6c>] (unwind_backtrace+0x0/0xf0) from [<8043ae34>] (panic+0x6c/0xe0)
[ 302.715187] [<8043ae34>] (panic+0x6c/0xe0) from [<8006ce00>] (die+0x2b4/0x304)
[ 302.722478] [<8006ce00>] (die+0x2b4/0x304) from [<8006ed6c>] (__do_kernel_fault+0x64/0x84)
[ 302.730774] [<8006ed6c>] (__do_kernel_fault+0x64/0x84) from [<8043f2fc>] (do_translation_fault+0x88/0x94)
[ 302.740390] [<8043f2fc>] (do_translation_fault+0x88/0x94) from [<800692c8>] (do_DataAbort+0x34/0x94)
[ 302.749569] [<800692c8>] (do_DataAbort+0x34/0x94) from [<8043d3ac>] (__dabt_svc+0x4c/0x60)
[ 302.757837] Exception stack(0xcffe5e48 to 0xcffe5e90)
[ 302.762904] 5e40: 00000680 a0000013 cffe5e60 000000c1 00000000 00000000
[ 302.771095] 5e60: cffe5ecf cf38772c 00000680 00000000 c3061588 00000000 00000000 cffe5e90
[ 302.779276] 5e80: 8009ad60 7f0870b4 60000013 ffffffff
[ 302.784345] [<8043d3ac>] (__dabt_svc+0x4c/0x60) from [<7f0870b4>] (0x7f0870b4)
4. 直接调用 dump_stack /show_stack / 等
比如：sched_show_task
wpa_supplican D 8043a3b4 0 249571 0x00080001 ______//24957 是wpa_supplicant 的PID
[<8043a3b4>] (schedule+0x2a4/0x324) from [<8043aa2c>] (schedule_timeout+0x18/0x1fc)
[<8043aa2c>] (schedule_timeout+0x18/0x1fc) from [<8043bae4>] (__down+0x7c/0xb4)
[<8043bae4>] (__down+0x7c/0xb4) from [<800aab40>] (down+0x3c/0x7c)
[<800aab40>] (down+0x3c/0x7c) from [<7f07c3e8>] (dhd_bus_rxctl+0x54/0x220 [bcm4330])
[<7f07c3e8>] (dhd_bus_rxctl+0x54/0x220 [bcm4330]) from [<7f07dd34>] (dhdcdc_cmplt+0x44/0x68 [bcm4330])
[<7f07dd34>] (dhdcdc_cmplt+0x44/0x68 [bcm4330]) from [<7f07e1a8>] (dhd_prot_ioctl+0x450/0x600 [bcm4330])
[<7f07e1a8>] (dhd_prot_ioctl+0x450/0x600 [bcm4330]) from [<7f06132c>] (dhd_wl_ioctl+0x34/0x48 [bcm4330])
[<7f06132c>] (dhd_wl_ioctl+0x34/0x48 [bcm4330]) from [<7f059820>] (dhd_ioctl_entry+0x3c0/0x4b0 [bcm4330])
[<7f059820>] (dhd_ioctl_entry+0x3c0/0x4b0 [bcm4330]) from [<7f0630c4>] (dev_wlc_ioctl+0xf0/0x150 [bcm4330])
[<7f0630c4>] (dev_wlc_ioctl+0xf0/0x150 [bcm4330]) from [<7f064238>] (wl_iw_mlme+0x64/0x80 [bcm4330])
[<7f064238>] (wl_iw_mlme+0x64/0x80 [bcm4330]) from [<8042ec68>] (ioctl_standard_call+0x2a0/0x388)
[<8042ec68>] (ioctl_standard_call+0x2a0/0x388) from [<8042ee00>] (wext_handle_ioctl+0xb0/0x1f4)
[<8042ee00>] (wext_handle_ioctl+0xb0/0x1f4) from [<80358a1c>] (dev_ioctl+0x754/0x784)
[<80358a1c>] (dev_ioctl+0x754/0x784) from [<80100814>] (vfs_ioctl+0x2c/0xac)
[<80100814>] (vfs_ioctl+0x2c/0xac) from [<80100ec8>] (do_vfs_ioctl+0x540/0x5a0)
[<80100ec8>] (do_vfs_ioctl+0x540/0x5a0) from [<80100f74>] (sys_ioctl+0x4c/0x6c)
[<80100f74>] (sys_ioctl+0x4c/0x6c) from [<80069960>] (ret_fast_syscall+0x0/0x30)