Date:	Thu, 16 Apr 2009 23:24:08 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 2/2] tracing/events/lockdep: move tracepoints within
 recursive protection




On Thu, 16 Apr 2009, Steven Rostedt wrote:
> I reverted my patch and added recursion detection, but I hit this nasty 
> bug:
> 
> ---------------------------------
> CPU 0 
> Modules linked in: radeon drm autofs4 hidp rfcomm l2cap bluetooth sunrpc 
> ip6t_REJECT ip6table_filter ip6_tables ipv6 sbs sbshc battery sg evbug 
> ide_cd_mod cdrom snd_intel8x0 snd_ac97_codec ac97_bus serio_raw 
> snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device 
> snd_pcm_oss snd_mixer_oss e1000 snd_pcm button floppy snd_timer i2c_i801 
> snd soundcore i2c_core snd_page_alloc ata_generic iTCO_wdt 
> iTCO_vendor_support pata_acpi e752x_edac edac_core pcspkr dm_snapshot 
> dm_zero dm_mirror dm_region_hash dm_log dm_mod ata_piix libata sd_mod 
> scsi_mod ext3 jbd ehci_hcd ohci_hcd uhci_hcd
> Pid: 3301, comm: pcscd Tainted: G        W  2.6.30-rc1 #1036 Precision 
> WorkStation 470    
> RIP: 0010:[<ffffffff803a59a2>]  [<ffffffff803a59a2>] strlen+0x2/0x20
> RSP: 0018:ffff880033d19c40  EFLAGS: 00010046
> RAX: 0000000000000000 RBX: ffffffff807617df RCX: ffffffff807617de
> RDX: ffff880033d19d48 RSI: ffff880033d19c58 RDI: 5b5b5b5b5b5b5b5b
> RBP: ffff880033d19ca8 R08: 5b5b5b5b5b5b5b00 R09: 0000000000000000
> R10: 0000000000000003 R11: 0000000000000202 R12: 0000000000000000
> R13: 5b5b5b5b5b5b5b5b R14: ffffffff81269b62 R15: ffff880033d19d08
> FS:  0000000041b6f940(0063) GS:ffffffff80828000(0000) 
> knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: 00000000006aec88 CR3: 0000000033c02000 CR4: 00000000000006e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400
> Process pcscd (pid: 3301, threadinfo ffff880033d18000, task 
> ffff880033c48000)
> Stack:
>  ffffffff803a73e5 ffffffff81269b60 ffffffff8126ab60 0000000000000004
>  0000000affffffff ffffffffffffffff 0000000000000000 ffff880033d19c98
>  0000000000000246 ffff880033d19d08 0000000000000000 0000000000000000
> Call Trace:
>  [<ffffffff803a73e5>] ? vbin_printf+0x1b5/0x2e0
>  [<ffffffff802b77dc>] trace_vbprintk+0xbc/0x1c0
>  [<ffffffff802887fe>] ? lock_acquire+0x10e/0x120
>  [<ffffffff802bad29>] __trace_bprintk+0x79/0x80
>  [<ffffffff80228ed6>] ? ftrace_call+0x5/0x2b
>  [<ffffffff802b5dac>] ? tracing_record_cmdline+0x2c/0x100
>  [<ffffffff80284920>] ftrace_event_lock_acquire+0x60/0x70
>  [<ffffffff802887fe>] lock_acquire+0x10e/0x120
>  [<ffffffff8030a744>] ? dput+0x64/0x170
>  [<ffffffff80610336>] _spin_lock+0x36/0x70
>  [<ffffffff8030a744>] ? dput+0x64/0x170
>  [<ffffffff8039f98f>] ? _atomic_dec_and_lock+0x1f/0x50
>  [<ffffffff8030a744>] dput+0x64/0x170
>  [<ffffffff80315215>] dcache_dir_close+0x15/0x20
>  [<ffffffff802f8a51>] __fput+0xd1/0x230
>  [<ffffffff802f8f0b>] fput+0x1b/0x30
>  [<ffffffff802f566d>] filp_close+0x5d/0x90
>  [<ffffffff802f5746>] sys_close+0xa6/0x110
>  [<ffffffff802291af>] system_call_fastpath+0x16/0x1b
> Code: 01 48 83 f9 ff 0f 1f 40 00 74 0b 48 83 c7 01 0f b6 07 84 c0 75 e3 c9 
> 31 c0 c3 c9 48 89 f8 c3 66 2e 0f 1f 84 00 00 00 00 00 31 c0 <80> 3f 00 55 
> 48 89 e5 74 11 48 89 f8 66 90 48 83 c0 01 80 38 00 
> RIP  [<ffffffff803a59a2>] strlen+0x2/0x20
>  RSP <ffff880033d19c40>
> 
> 
> Doing an objdump:
> 
> ffffffff803a59a0 <strlen>:
> ffffffff803a59a0:       31 c0                   xor    %eax,%eax
> ffffffff803a59a2:       80 3f 00                cmpb   $0x0,(%rdi)
> ffffffff803a59a5:       55                      push   %rbp
> ffffffff803a59a6:       48 89 e5                mov    %rsp,%rbp
> ffffffff803a59a9:       74 11                   je     ffffffff803a59bc <strlen+0x1c>
> 
> The %rdi is: 5b5b5b5b5b5b5b5b
> 
> Either there's a bug in the vbin_printf, or we have some crazy lock->name?
> 
> TRACE_FORMAT(lock_acquire,
>         TP_PROTO(struct lockdep_map *lock, unsigned int subclass,
>                 int trylock, int read, int check,
>                 struct lockdep_map *next_lock, unsigned long ip),
>         TP_ARGS(lock, subclass, trylock, read, check, next_lock, ip),
>         TP_FMT("%s%s%s", trylock ? "try " : "",
>                 read ? "read " : "", lock->name)
>         );
> 
> I'll continue to look into this, and perhaps reboot and see what other 
> nasties I hit.

OK, I think I figured this bug out. This is a lockdep issue with respect 
to tracepoints.

The trace points in lockdep are called all the time. Outside the lockdep 
logic. But if lockdep were to trigger an error / warning (which this run 
did) we might be in trouble. For new locks, like the dentry->d_lock, that 
are created, they will not get a name:

void lockdep_init_map(struct lockdep_map *lock, const char *name,
                      struct lock_class_key *key, int subclass)
{
        if (unlikely(!debug_locks))
                return;

When a problem is found by lockdep, debug_locks becomes false. Thus we 
stop allocating names for locks. This dentry->d_lock I had, now has no 
name. Worse yet, I have CONFIG_DEBUG_VM set, that scrambles non 
initialized memory. Thus, when the trace point was hit, it had junk for 
the lock->name, and the machine crashed.


-- Steve

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