bug: ftrace & lockdep badness

[Heiko Carstens <heiko.carstens@de.ibm.com>]

Hi Steven,

enabling preemptirqsoff tracing gives me this on a lockdep enabled kernel:

Badness at kernel/lockdep.c:2894
Modules linked in:
CPU: 0 Not tainted 2.6.28-rc3-00031-ga75952b-dirty #72
Process sshd (pid: 2542, task: 000000003397d7a8, ksp: 0000000033abb618)
Krnl PSW : 0404c00180000000 0000000000078770 (check_flags+0x12c/0x160)
           R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 EA:3
Krnl GPRS: 0000000000000000 0000000000dcda88 0000000000000000 000000000000000f
           0000000000000000 0000000000000000 0000000000000002 0000000000000000
           0000000000000000 0000000000000000 000000003fe7d428 0000000033abb170
           0000000000557fb8 000000000038b210 0000000000078758 0000000033abb170
Krnl Code: 0000000000078764: bf2f1000           icm     %r2,15,0(%r1)
           0000000000078768: a774ffa7           brc     7,786b6
           000000000007876c: a7f40001           brc     15,7876e
          >0000000000078770: a7f4ffa3           brc     15,786b6
           0000000000078774: c0e5000a53ea       brasl   %r14,1c2f48
           000000000007877a: 1222               ltr     %r2,%r2
           000000000007877c: a784000b           brc     8,78792
           0000000000078780: c010006aa984       larl    %r1,dcda88
Call Trace:
([<0000000033abb188>] 0x33abb188)
 [<000000000007e76a>] lock_acquire+0x52/0xbc
 [<000000000037618c>] _spin_lock_irqsave+0x6c/0xb0
 [<000000000009d52e>] ring_buffer_reset_cpu+0x6e/0x114
 [<00000000000a46c0>] tracing_reset+0x84/0xe8
 [<00000000000a5632>] trace_preempt_off+0x112/0x178
 [<0000000000378888>] add_preempt_count+0xa4/0x130
 [<00000000000539e0>] __local_bh_disable+0x54/0xcc
 [<0000000000053a82>] local_bh_disable+0x2a/0x38
 [<0000000000376002>] _spin_lock_bh+0x2e/0xa4
 [<00000000002a1baa>] lock_sock_nested+0x4a/0x118
 [<00000000002e9e86>] tcp_sendmsg+0x42/0xdc0
 [<000000000029cd3c>] sock_aio_write+0x130/0x180
 [<00000000000ea36c>] do_sync_write+0xdc/0x134
 [<00000000000eafe8>] vfs_write+0x154/0x15c
 [<00000000000eb106>] sys_write+0x52/0xa8
 [<000000000002a950>] sys32_write+0x2c/0x4c
 [<0000000000026f80>] sysc_noemu+0x10/0x16
 [<0000000077c5c186>] 0x77c5c186
INFO: lockdep is turned off.

Seems to be that the following happens:

__local_bh_disable() calls add_preempt_count() which adds something to
preempt_count().
After this addition softirq_count() will return something != 0.
However current->softirqs_enabled is still 1 since we haven't called
trace_softirqs_off() yet.
But we call trace_preempt_off() instead which will grab a lock in the
ringbuffer code and that will trigger this check in lockdep.c:

        if (!hardirq_count()) {
                if (softirq_count())
                        DEBUG_LOCKS_WARN_ON(current->softirqs_enabled);

I assume this is a known bug. Is there any fix available?