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SubjectRe: [PATCH] kernel, add bug_on_warn


On 10/20/2014 06:24 PM, Andrew Morton wrote:
> On Mon, 20 Oct 2014 08:00:20 -0400 Prarit Bhargava <prarit@redhat.com> wrote:
>
>> There have been several times where I have had to rebuild a kernel to
>> cause a panic when hitting a WARN() in the code in order to get a crash
>> dump from a system. Sometimes this is easy to do, other times (such as
>> in the case of a remote admin) it is not trivial to send new images to the
>> user.
>>
>> A much easier method would be a switch to change the WARN() over to a
>> BUG(). This makes debugging easier in that I can now test the actual
>> image the WARN() was seen on and I do not have to engage in remote
>> debugging.
>>
>> This patch adds a bug_on_warn kernel parameter, which calls BUG() in the
>> warn_slowpath_common() path. The function will still print out the
>> location of the warning.
>>
>> Successfully tested by me.
>
> Looks nice and simple and useful. However I suspect you're exclusively
> focussed on "I want a crash dump" and things haven't been fully thought
> through.
>
> - Do you have any example WARN->BUG console output at hand? I'd like
> to check for missing or duplicated info.

Yep, here you go, with some additional annotation notes from me. The first
line below is from the WARN_ON() to output the WARN_ON()'s location. After
that, we hit the new BUG() call.

WARNING: CPU: 27 PID: 3204 at
/home/rhel7/redhat/debug/dummy-module/dummy-module.c:25 init_dummy+0x28/0x30
[dummy_module]()
bug_on_warn set, calling BUG()...
------------[ cut here ]------------
kernel BUG at kernel/panic.c:434!
invalid opcode: 0000 [#1] SMP
Modules linked in: dummy_module(OE+) sg nfsv3 rpcsec_gss_krb5 nfsv4
dns_resolver nfs fscache cfg80211 rfkill x86_pkg_temp_thermal intel_powerclamp
coretemp kvm_intel kvm crct10dif_pclmul crc32_pclmul crc32c_intel
ghash_clmulni_intel igb iTCO_wdt aesni_intel iTCO_vendor_support lrw gf128mul
sb_edac ptp edac_core glue_helper lpc_ich ioatdma pcspkr ablk_helper pps_core
i2c_i801 mfd_core cryptd dca shpchp ipmi_si wmi ipmi_msghandler acpi_cpufreq
nfsd auth_rpcgss nfs_acl lockd grace sunrpc xfs libcrc32c sr_mod cdrom sd_mod
mgag200 syscopyarea sysfillrect sysimgblt i2c_algo_bit drm_kms_helper isci ttm
drm libsas ahci libahci scsi_transport_sas libata i2c_core dm_mirror
dm_region_hash dm_log dm_mod
CPU: 27 PID: 3204 Comm: insmod Tainted: G OE 3.17.0+ #19
Hardware name: Intel Corporation S2600CP/S2600CP, BIOS
RMLSDP.86I.00.29.D696.1311111329 11/11/2013
task: ffff880034e75160 ti: ffff8807fc5ac000 task.ti: ffff8807fc5ac000
RIP: 0010:[<ffffffff81076b81>] [<ffffffff81076b81>] warn_slowpath_common+0xc1/0xd0
RSP: 0018:ffff8807fc5afc68 EFLAGS: 00010246
RAX: 0000000000000021 RBX: ffff8807fc5afcb0 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff88081efee5f8 RDI: ffff88081efee5f8
RBP: ffff8807fc5afc98 R08: 0000000000000096 R09: 0000000000000000
R10: 0000000000000711 R11: ffff8807fc5af93e R12: ffffffffa0424070
R13: 0000000000000019 R14: ffffffffa0423068 R15: 0000000000000009
FS: 00007f2d4b034740(0000) GS:ffff88081efe0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f2d4a99f3c0 CR3: 00000007fd88b000 CR4: 00000000001407e0
Stack:
ffff8807fc5afcb8 ffffffff8199f020 ffff88080e396160 0000000000000000
ffffffffa0423040 ffffffffa0425000 ffff8807fc5afd08 ffffffff81076be5
0000000000000008 ffffffffa0424053 ffff880700000018 ffff8807fc5afd18
Call Trace:
[<ffffffffa0423040>] ? dummy_greetings+0x40/0x40 [dummy_module]
[<ffffffff81076be5>] warn_slowpath_fmt+0x55/0x70
[<ffffffffa0423068>] init_dummy+0x28/0x30 [dummy_module]
[<ffffffff81002144>] do_one_initcall+0xd4/0x210
[<ffffffff811b52c2>] ? __vunmap+0xc2/0x110
[<ffffffff810f8889>] load_module+0x16a9/0x1b30
[<ffffffff810f3d30>] ? store_uevent+0x70/0x70
[<ffffffff810f49b9>] ? copy_module_from_fd.isra.44+0x129/0x180
[<ffffffff810f8ec6>] SyS_finit_module+0xa6/0xd0
[<ffffffff8166ce29>] system_call_fastpath+0x12/0x17
Code: c4 08 5b 41 5c 41 5d 41 5e 41 5f 5d c3 48 c7 c7 20 42 8a 81 31 c0 e8 fc
80 5e 00 eb 80 48 c7 c7 78 42 8a 81 31 c0 e8 ec 80 5e 00 <0f> 0b 66 66 66 66 2e
0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 55
RIP [<ffffffff81076b81>] warn_slowpath_common+0xc1/0xd0
RSP <ffff8807fc5afc68>
---[ end trace 428218934a12088b ]---
>
> - Did you consider permitting this to be tweaked at runtime via
> /proc? Sometimes we get pesky WARNs at boot time and having runtime
> alteration would permit the user to prevent those from tripping a
> BUG.
>

I did actually, but I was wondering how people liked the idea before I looked
at the /proc implementation. It's pretty much the same as panic_on_oops, so
it's not difficult to do.

> - Also, perhaps bug_on_warn should be single-shot: clear itself after
> it has triggered one BUG. Because once the kernel has gone
> WARN->BUG, it's probably messed up and is likely to trigger more
> WARNs. Also, the kernel might generate many WARNs for the same
> issue.

Okay, I'll add that.

>
>> --- a/Documentation/kernel-parameters.txt
>> +++ b/Documentation/kernel-parameters.txt
>> @@ -553,6 +553,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>> bttv.pll= See Documentation/video4linux/bttv/Insmod-options
>> bttv.tuner=
>>
>> + bug_on_warn BUG() instead of WARN()
>
> There's no mention here that this feature is mainly aimed at generating
> a crash dump. How do we tell the people who aren't reading this email
> thread (ie: all of humanity except you and me ;)) that this feature
> even exists? Is there crash dump documentation that we can update?
>

I'll look into this too.

P.
>


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