lockdep warning after recent cleanup in console code

["Shevchenko, Andriy" <andriy.shevchenko@intel.com>]

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Hei!

During weekend the linux-next was being broken by introducing a lockdep
warning in the console code

[    0.000000] BIOS-e820: [mem 0x00000000e0000000-0x00000000ffffffff]
reserved
[    0.000000]
[    0.000000] =============================================
[    0.000000] [ INFO: possible recursive locking detected ]
[    0.000000] 3.15.0-rc2-next-20140428-00030-gd3550d8 #38 Not tainted
[    0.000000] ---------------------------------------------
[    0.000000] swapper/0 is trying to acquire lock:
[    0.000000]  (console_lock){......}, at: [<c10728cc>]
register_console+0x15e/0x295
[    0.000000]
[    0.000000] but task is already holding lock:
[    0.000000]  (console_lock){......}, at: [<c1071e7a>] vprintk_emit
+0x381/0x3ea
[    0.000000]
[    0.000000] other info that might help us debug this:
[    0.000000]  Possible unsafe locking scenario:
[    0.000000]
[    0.000000]        CPU0
[    0.000000]        ----
[    0.000000]   lock(console_lock);
[    0.000000]   lock(console_lock);
[    0.000000]
[    0.000000]  *** DEADLOCK ***
[    0.000000]
[    0.000000]  May be due to missing lock nesting notation
[    0.000000]
[    0.000000] 3 locks held by swapper/0:
[    0.000000]  #0:  (console_lock){......}, at: [<c1071e7a>]
vprintk_emit+0x381/0x3ea
[    0.000000]  #1:  (console_lock){......}, at: [<c1071e7a>]
vprintk_emit+0x381/0x3ea
[    0.000000]  #2:  (console_lock){......}, at: [<c1071e7a>]
vprintk_emit+0x381/0x3ea
[    0.000000]
[    0.000000] stack backtrace:
[    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted
3.15.0-rc2-next-20140428-00030-gd3550d8 #38
[    0.000000]  00000000 00000000 c186de1c c15878ae c1daf840 c186de88
c106c154 c1794d9d
[    0.000000]  c17958f7 c1794c9c 0000002a 00000000 00000000 c1daf840
c1983700 00000080
[    0.000000]  04002001 c187a0b0 00000000 c1879ba0 c1879b40 c1879b40
00200046 00000000
[    0.000000] Call Trace:
[    0.000000]  [<c15878ae>] dump_stack+0x49/0x73
[    0.000000]  [<c106c154>] __lock_acquire+0xb31/0xc7c
[    0.000000]  [<c106c33b>] lock_acquire+0x9c/0x111
[    0.000000]  [<c10728cc>] ? register_console+0x15e/0x295
[    0.000000]  [<c10707dd>] console_lock+0x41/0x46
[    0.000000]  [<c10728cc>] ? register_console+0x15e/0x295
[    0.000000]  [<c10728cc>] register_console+0x15e/0x295
[    0.000000]  [<c1582f80>] early_console_register+0x33/0x35
[    0.000000]  [<c1910411>] setup_early_printk+0x144/0x151
[    0.000000]  [<c19013f6>] do_early_param+0x41/0x73
[    0.000000]  [<c19013b5>] ? loglevel+0x2c/0x2c
[    0.000000]  [<c104c766>] parse_args+0x24f/0x307
[    0.000000]  [<c19013b5>] ? loglevel+0x2c/0x2c
[    0.000000]  [<c190169d>] parse_early_options+0x1c/0x21
[    0.000000]  [<c190169d>] ? parse_early_options+0x1c/0x21
[    0.000000]  [<c19013b5>] ? loglevel+0x2c/0x2c
[    0.000000]  [<c19016cc>] parse_early_param+0x2a/0x36
[    0.000000]  [<c19042c0>] setup_arch+0x3c9/0xc55
[    0.000000]  [<c1071ebe>] ? vprintk_emit+0x3c5/0x3ea
[    0.000000]  [<c1584869>] ? printk+0x28/0x2d
[    0.000000]  [<c1901754>] start_kernel+0x72/0x305
[    0.000000]  [<c19012b4>] i386_start_kernel+0x82/0x86
[    0.000000] bootconsole [earlyhsu0] enabled

Bisecting shows the culprit (which I guessed before), namely commit
5dc90cb49691755faaad2a395d297d0162075eca ("printk: enable interrupts 
before calling console_trylock_for_printk()").

Following patch helps me, but I'm not familiar with console locking
scheme, I believe there is a better solution.

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1700,8 +1700,10 @@ asmlinkage int vprintk_emit(int facility, int
level,
         * The release will print out buffers and wake up /dev/kmsg and
syslog()
         * users.
         */
+       lockdep_off();
        if (console_trylock_for_printk())
                console_unlock();
+       lockdep_on();
        preempt_enable();
 
        return printed_len;


-- 
Andy Shevchenko <andriy.shevchenko@intel.com>
Intel Finland Oy
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Intel Finland Oy
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