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

[Eric Dumazet <edumazet@google.com>]

On Wed, Dec 11, 2019 at 10:38 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Wed, Dec 11, 2019 at 10:02 PM Paul E. McKenney <paulmck@kernel.org> 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);


Also add_timer() can benefit from the same hint, since it seems inlined as well.

(untested patch)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4820823515e9..568564ae3597 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
@@ -1133,7 +1134,7 @@ EXPORT_SYMBOL(timer_reduce);
 void add_timer(struct timer_list *timer)
 {
        BUG_ON(timer_pending(timer));
-       mod_timer(timer, timer->expires);
+       __mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING);
 }
 EXPORT_SYMBOL(add_timer);

@@ -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);