Re: linux-next: build failure after merge of the rcu tree

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Jan 24, 2024 at 10:49:54AM +0100, Jiri Wiesner wrote:
> On Wed, Jan 24, 2024 at 03:17:43PM +1100, Stephen Rothwell wrote:
> > After merging the rcu tree, today's linux-next build (i386 defconfig)
> > failed like this:
> > In file included from include/linux/dev_printk.h:14,
> >                  from include/linux/device.h:15,
> >                  from kernel/time/clocksource.c:10:
> > kernel/time/clocksource.c: In function 'clocksource_watchdog':
> > kernel/time/clocksource.c:103:34: error: integer overflow in expression of type 'long int' results in '-1619276800' [-Werror=overflow]
> >   103 |                                  * NSEC_PER_SEC / HZ)
> >       |                                  ^
> > Caused by commit
> >   1a4545025600 ("clocksource: Skip watchdog check for large watchdog intervals")
> > I have used the rcu tree from next-20240123 for today.
> 
> This particular patch is still beging discussed on the LKML. This is the 
> latest submission with improved variable naming, increased threshold and 
> changes to the log and the warning message (as proposed by tglx):
> https://lore.kernel.org/lkml/20240122172350.GA740@incl/
> Especially the change to the message is important. I think this message 
> will be commonplace on 8 NUMA node (and larger) machines. If there is 
> anything else I can do to assist please let me know.

Here is the offending #define:

#define WATCHDOG_INTR_MAX_NS	((WATCHDOG_INTERVAL + (WATCHDOG_INTERVAL >> 1))\
				 * NSEC_PER_SEC / HZ)

The problem is that these things are int or long, and on i386, that
is only 32 bits.  NSEC_PER_SEC is one billion, and WATCHDOG_INTERVAL
is often 1000, which overflows.  The division by HZ gets this back in
range at about 1.5x10^9.

So this computation must be done in 64 bits even on 32-bit systems.
My thought would be a cast to u64, then back to long for the result.

Whatever approach, Jiri, would you like to send an updated patch?

In the meantime, I will rebase to exclude this one from -next.

							Thanx, Paul