Re: [PATCH 2/2] posix-timers: Use RCU in posix_timer_add()

[Eric Dumazet]

On Fri, Feb 14, 2025 at 5:59 PM David Laight
<david.laight.linux@xxxxxxxxx> wrote:
>
> On Fri, 14 Feb 2025 13:59:11 +0000
> Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> > If many posix timers are hashed in posix_timers_hashtable,
> > hash_lock can be held for long durations.
> >
> > This can be really bad in some cases as Thomas
> > explained in https://lore.kernel.org/all/87ednpyyeo.ffs@tglx/
> >
> > We can perform all searches under RCU, then acquire
> > the lock only when there is a good chance to need it,
> > and after cpu caches were populated.
> >
> > I also added a cond_resched() in the possible long loop.
> >
> > Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> > ---
> >  kernel/time/posix-timers.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
> > index 204a351a2fd3..dd2f9016d3dc 100644
> > --- a/kernel/time/posix-timers.c
> > +++ b/kernel/time/posix-timers.c
> > @@ -112,7 +112,19 @@ static int posix_timer_add(struct k_itimer *timer)
> >
> >               head = &posix_timers_hashtable[hash(sig, id)];
> >
> > +             rcu_read_lock();
> > +             if (__posix_timers_find(head, sig, id)) {
> > +                     rcu_read_unlock();
> > +                     cond_resched();
> > +                     continue;
> > +             }
> > +             rcu_read_unlock();
> >               spin_lock(&hash_lock);
> > +             /*
> > +              * We must perform the lookup under hash_lock protection
> > +              * because another thread could have used the same id.
> > +              * This is very unlikely, but possible.
> > +              */
>
> If next_posix_timer_id is 64bit (so can't wrap) I think you can compare the
> (unmasked by MAX_INT) value being used with the current value.
> If the difference is small (well less than MAX_INT) I don't think you need
> the rescan.
> (Not going to help 32bit - but who cares :-)

I just noticed the rescan is racy anyway, because when the other threads add
a timer, the timer->it_signal and timer->it_id are temporarily zero.

There is a small race window.

We can set timer->it_id earlier [1], but not timer->it_signal

More work is needed :)

[1]

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index dd2f9016d3dc..59ff75c81cff 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -126,6 +126,7 @@ static int posix_timer_add(struct k_itimer *timer)
                 * This is very unlikely, but possible.
                 */
                if (!__posix_timers_find(head, sig, id)) {
+                       timer->it_id = (timer_t)id;
                        hlist_add_head_rcu(&timer->t_hash, head);
                        spin_unlock(&hash_lock);
                        return id;
@@ -428,7 +429,6 @@ static int do_timer_create(clockid_t which_clock,
struct sigevent *event,
                return new_timer_id;
        }

-       new_timer->it_id = (timer_t) new_timer_id;
        new_timer->it_clock = which_clock;
        new_timer->kclock = kc;
        new_timer->it_overrun = -1LL;