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

[Eric Dumazet]

On Wed, Dec 11, 2019 at 10:02 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> 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?

Ah... the READ_ONCE() apparently turns off the compiler ability to
infer that this branch should not be taken.

Since __mod_timer() is inlined we could perhaps add a new option

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4820823515e9..8bbce552568b 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -944,6 +944,7 @@ static struct timer_base *lock_timer_base(struct
timer_list *timer,

 #define MOD_TIMER_PENDING_ONLY         0x01
 #define MOD_TIMER_REDUCE               0x02
+#define MOD_TIMER_NOTPENDING           0x04

 static inline int
 __mod_timer(struct timer_list *timer, unsigned long expires, unsigned
int options)
@@ -960,7 +961,7 @@ __mod_timer(struct timer_list *timer, unsigned
long expires, unsigned int option
         * the timer is re-modified to have the same timeout or ends up in the
         * same array bucket then just return:
         */
-       if (timer_pending(timer)) {
+       if (!(options & MOD_TIMER_NOTPENDING) && timer_pending(timer)) {
                /*
                 * The downside of this optimization is that it can result in
                 * larger granularity than you would get from adding a new
@@ -1891,7 +1892,7 @@ signed long __sched schedule_timeout(signed long timeout)

        timer.task = current;
        timer_setup_on_stack(&timer.timer, process_timeout, 0);
-       __mod_timer(&timer.timer, expire, 0);
+       __mod_timer(&timer.timer, expire, MOD_TIMER_NOTPENDING);
        schedule();
        del_singleshot_timer_sync(&timer.timer);