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

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 11 Dec 2019 22:02:00 -0800

On Thu, Dec 12, 2019 at 04:06:22PM +1100, Stephen Rothwell wrote:
> Hi all,
> 
> After merging the rcu (I think) tree, today's linux-next build (x86_64
> allnoconfig) produced this warning:
> 
> kernel/time/timer.c: In function 'schedule_timeout':
> kernel/time/timer.c:969:20: warning: 'timer.expires' may be used uninitialized in this function [-Wmaybe-uninitialized]
>   969 |   long diff = timer->expires - expires;
>       |               ~~~~~^~~~~~~~~
> 
> Introduced by (bisected to) commit
> 
>   c4127fce1d02 ("timer: Use hlist_unhashed_lockless() in timer_pending()")
> 
> x86_64-linux-gnu-gcc (Debian 9.2.1-21) 9.2.1 20191130

Well, if the timer is pending, then ->expires has to have been
initialized, but off where the compiler cannot see it, such as during a
previous call to __mod_timer().  And the change may have made it harder
for the compiler to see all of these relationships, but...

I don't see this warning with gcc version 7.4.0.  Just out of curiosity,
what are you running, Stephen?

Eric, any thoughts for properly educating the compiler on this one?

							Thanx, Paul