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

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Date:   Tue, 8 Jun 2021 15:48:22 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     kernel test robot <lkp@...el.com>
Cc:     John Ogness <john.ogness@...utronix.de>, kbuild-all@...ts.01.org,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>
Subject: Re: [PATCH next v2 1/2] dump_stack: move cpu lock to printk.c

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@...el.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