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

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Date:   Thu, 10 Jun 2021 15:26:15 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>, kernel test robot <lkp@...el.com>
Cc:     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 2021-06-08, Petr Mladek <pmladek@...e.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