Re: [PATCH next v2 1/2] dump_stack: move cpu lock to printk.c

Re: [PATCH next v2 1/2] dump_stack: move cpu lock to printk.c

[kernel test robot]

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Hi John,

I love your patch! Perhaps something to improve:

[auto build test WARNING on next-20210607]

url:    https://github.com/0day-ci/linux/commits/John-Ogness/introduce-printk-cpu-lock/20210608-040454
base:    7f09e895a7f3e0af63bf9ec6c7c22893ec7e6c8e
config: mips-randconfig-r036-20210607 (attached as .config)
compiler: mips-linux-gcc (GCC) 9.3.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/136bcc2980e636b2ae156ca63fbe95c713e44c1b
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review John-Ogness/introduce-printk-cpu-lock/20210608-040454
        git checkout 136bcc2980e636b2ae156ca63fbe95c713e44c1b
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=mips 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

   lib/dump_stack.c: In function 'dump_stack_lvl':
>> lib/dump_stack.c:107:2: warning: 'lock_flag' is used uninitialized in this function [-Wuninitialized]
     107 |  printk_cpu_unlock_irqrestore(lock_flag, irq_flags);
         |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


vim +/lock_flag +107 lib/dump_stack.c

    90	
    91	/**
    92	 * dump_stack - dump the current task information and its stack trace
    93	 *
    94	 * Architectures can override this implementation by implementing its own.
    95	 */
    96	asmlinkage __visible void dump_stack_lvl(const char *log_lvl)
    97	{
    98		unsigned long irq_flags;
    99		bool lock_flag;
   100	
   101		/*
   102		 * Permit this cpu to perform nested stack dumps while serialising
   103		 * against other CPUs
   104		 */
   105		printk_cpu_lock_irqsave(&lock_flag, &irq_flags);
   106		__dump_stack(log_lvl);
 > 107		printk_cpu_unlock_irqrestore(lock_flag, irq_flags);
   108	}
   109	EXPORT_SYMBOL(dump_stack_lvl);
   110	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

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Re: [PATCH next v2 1/2] dump_stack: move cpu lock to printk.c

[Petr Mladek]

On Tue 2021-06-08 10:43:46, kernel test robot wrote:
> Hi John,
> 
> I love your patch! Perhaps something to improve:
> 
> [auto build test WARNING on next-20210607]
> 
> url:    https://github.com/0day-ci/linux/commits/John-Ogness/introduce-printk-cpu-lock/20210608-040454
> base:    7f09e895a7f3e0af63bf9ec6c7c22893ec7e6c8e
> config: mips-randconfig-r036-20210607 (attached as .config)
> compiler: mips-linux-gcc (GCC) 9.3.0
> reproduce (this is a W=1 build):
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # https://github.com/0day-ci/linux/commit/136bcc2980e636b2ae156ca63fbe95c713e44c1b
>         git remote add linux-review https://github.com/0day-ci/linux
>         git fetch --no-tags linux-review John-Ogness/introduce-printk-cpu-lock/20210608-040454
>         git checkout 136bcc2980e636b2ae156ca63fbe95c713e44c1b
>         # save the attached .config to linux build tree
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=mips 
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
> 
> All warnings (new ones prefixed by >>):
> 
>    lib/dump_stack.c: In function 'dump_stack_lvl':
> >> lib/dump_stack.c:107:2: warning: 'lock_flag' is used uninitialized in this function [-Wuninitialized]
>      107 |  printk_cpu_unlock_irqrestore(lock_flag, irq_flags);
>          |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Interesting. I am curious that it does not complain also about
irq_flags. But it is possible the it reports only the first problem.

Anyway, we will likely need to do some trickery via #define to tell
the compiler that the value is set. I mean to do similar thing as:

#define raw_local_irq_save(flags)			\
	do {						\
		typecheck(unsigned long, flags);	\
		flags = arch_local_irq_save();		\
	} while (0)


In our case, it might look like:

#define printk_cpu_lock_irqsave(lock_nested, irq_flags)		\
	do {							\
		local_irq_save(irq_flags);			\
		typecheck(bool, lock_nested);			\
		lock_nested = __printk_cpu_lock(irq_flags);	\
	} while (0)


then we would need to do in __prink_cpu_lock(unsigned long irq_flags)


	} else {
		local_irq_restore(irq_flags);

		/*
		 * Wait for the lock to release before jumping to cmpxchg()
		 * in order to mitigate the thundering herd problem.
		 */
		do {
			cpu_relax();
		} while (atomic_read(&printk_cpulock_owner) != -1);

		local_irq_save(irq_flags)
		goto retry;
	}

Best Regards,
Petr

Re: [PATCH next v2 1/2] dump_stack: move cpu lock to printk.c

[John Ogness]

On 2021-06-08, Petr Mladek <pmladek@suse.com> wrote:
>>    lib/dump_stack.c: In function 'dump_stack_lvl':
>> >> lib/dump_stack.c:107:2: warning: 'lock_flag' is used uninitialized in this function [-Wuninitialized]
>>      107 |  printk_cpu_unlock_irqrestore(lock_flag, irq_flags);
>>          |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Interesting. I am curious that it does not complain also about
> irq_flags. But it is possible the it reports only the first problem.

Strangely enough, if I set a value for @lock_flag, it is happy and does
not complain about @irq_flags. Probably a compiler oversight.

> Anyway, we will likely need to do some trickery via #define to tell
> the compiler that the value is set.

This is on ARCH=mips and !CONFIG_SMP. So the value is _not_ getting
set. (The static inline function does nothing.)

By changing printk_cpu_unlock_irqrestore() to use pointers:

    static inline void printk_cpu_unlock_irqrestore(bool *lock_flag, unsigned long *irq_flags)

then the warning disappears. Indeed, by not using pointers on unlock,
technically data is copied that was never initialized. I thought maybe
the compiler would optimize all that out, but it seems that it does not.

I have no problems using pointers for unlock(). It was strange using
pointers for lock(), but not for unlock() anyway.

Or would you prefer something else?

John Ogness