Re: [PATCH] lockdep: make print_lock_name() robust against non-existing lock_class

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

* Peter Zijlstra | 2015-04-16 17:35:03 [+0200]:

>On Thu, Apr 16, 2015 at 04:50:21PM +0200, Sebastian Andrzej Siewior wrote:
>
>> Andreas confirmed that it works for him on v3.18 with minor adjustment.
>> 
>> <---
>> +       struct held_lock lock = READ_ONCE(*hlock);
>> +       unsigned int class_idx = lock.class_idx;
>> --->
>> 
>
>I'm confused by the need for that. What was the failure with the
>proposed patch?

It was tested on v3.18, there might have been a change between v3.18 &
4.0. The patch as-is did no compile:

 in file included from arch/x86/include/asm/current.h:4:0,
                  from include/linux/mutex.h:13,
                  from kernel/locking/lockdep.c:29:
 kernel/locking/lockdep.c: In function ‘print_lock’:
 kernel/locking/lockdep.c:558:37: error: ‘typeof’ applied to a bit-field
   unsigned int class_idx = READ_ONCE(hlock->class_idx);
                                      ^
 include/linux/compiler.h:262:20: note: in definition of macro ‘READ_ONCE’
   ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
                     ^
 include/linux/compiler.h:262:11: error: cannot take address of bit-field ‘class_idx’
   ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
            ^
 kernel/locking/lockdep.c:558:27: note: in expansion of macro ‘READ_ONCE’ 
   unsigned int class_idx = READ_ONCE(hlock->class_idx);
                            ^
 include/linux/compiler.h:262:88: error: ‘sizeof’ applied to a bit-field
   ({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
 
                  ^
 kernel/locking/lockdep.c:558:27: note: in expansion of macro ‘READ_ONCE’ 
   unsigned int class_idx = READ_ONCE(hlock->class_idx);
                            ^
 scripts/Makefile.build:258: recipe for target 'kernel/locking/lockdep.o' failed 

Sebastian