linux-kernel - Re: kernel/time/clocksource-wdtest.c:47:22: warning: signed shift result (0x27BC86B00) requires 35 bits to represent, but 'long' only has 32 bits

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Message-ID: <20210704174310.GR4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Sun, 4 Jul 2021 10:43:10 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     kernel test robot <lkp@...el.com>
Cc:     clang-built-linux@...glegroups.com, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: kernel/time/clocksource-wdtest.c:47:22: warning: signed shift
 result (0x27BC86B00) requires 35 bits to represent, but 'long' only has 32
 bits

On Sun, Jul 04, 2021 at 05:42:36AM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   303392fd5c160822bf778270b28ec5ea50cab2b4
> commit: 1253b9b87e42ab6a3d5c2cb27af2bdd67d7e50ff clocksource: Provide kernel module to test clocksource watchdog
> date:   11 days ago
> config: mips-randconfig-r015-20210702 (attached as .config)
> compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 9eb613b2de3163686b1a4bd1160f15ac56a4b083)
> 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
>         # install mips cross compiling tool for clang build
>         # apt-get install binutils-mips-linux-gnu
>         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1253b9b87e42ab6a3d5c2cb27af2bdd67d7e50ff
>         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>         git fetch --no-tags linus master
>         git checkout 1253b9b87e42ab6a3d5c2cb27af2bdd67d7e50ff
>         # save the attached .config to linux build tree
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang 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 >>):
> 
> >> kernel/time/clocksource-wdtest.c:47:22: warning: signed shift result (0x27BC86B00) requires 35 bits to represent, but 'long' only has 32 bits [-Wshift-overflow]
>            .mult                   = TICK_NSEC << JIFFIES_SHIFT, /* details above */
>                                      ~~~~~~~~~ ^  ~~~~~~~~~~~~~
>    1 warning generated.

You have CONFIG_HZ_24=y and this test module assumes HZ > 100, as noted
just above the excerpt below:

/* Assume HZ > 100. */
#define JIFFIES_SHIFT   8

So this is expected behavior.

Would it help if I gave that comment some teeth, for example, as
shown at the end of this email?

> vim +/long +47 kernel/time/clocksource-wdtest.c
> 
>     39	
>     40	static struct clocksource clocksource_wdtest_jiffies = {
>     41		.name			= "wdtest-jiffies",
>     42		.rating			= 1, /* lowest valid rating*/
>     43		.uncertainty_margin	= TICK_NSEC,
>     44		.read			= wdtest_jiffies_read,
>     45		.mask			= CLOCKSOURCE_MASK(32),
>     46		.flags			= CLOCK_SOURCE_MUST_VERIFY,
>   > 47		.mult			= TICK_NSEC << JIFFIES_SHIFT, /* details above */
>     48		.shift			= JIFFIES_SHIFT,
>     49		.max_cycles		= 10,
>     50	};
>     51	

------------------------------------------------------------------------

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index e1856030fa66..8f0ad6e4183c 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2573,6 +2573,7 @@ config TEST_FPU
 config TEST_CLOCKSOURCE_WATCHDOG
 	tristate "Test clocksource watchdog in kernel space"
 	depends on CLOCKSOURCE_WATCHDOG
+	depends on HZ >= 100
 	help
 	  Enable this option to create a kernel module that will trigger
 	  a test of the clocksource watchdog.  This module may be loaded