Messages in this thread | | | Date | Tue, 4 Sep 2012 16:53:21 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 2/3] rcu: Disallow callback registry on offline CPUs |
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On Fri, Aug 31, 2012 at 09:21:30AM -0700, Josh Triplett wrote: > On Thu, Aug 30, 2012 at 12:03:02PM -0700, Paul E. McKenney wrote: > > From: "Paul E. McKenney" <paul.mckenney@linaro.org> > > > > Posting a callback after the CPU_DEAD notifier effectively leaks > > that callback unless/until that CPU comes back online. Silence is > > unhelpful when attempting to track down such leaks, so this commit emits > > a WARN_ON_ONCE() and unconditionally leaks the callback when an offline > > CPU attempts to register a callback. The rdp->nxttail[RCU_NEXT_TAIL] is > > set to NULL in the CPU_DEAD notifier and restored in the CPU_UP_PREPARE > > notifier, allowing _call_rcu() to determine exactly when posting callbacks > > is illegal. > > > > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > One suggestion below; with or without that change: > > Reviewed-by: Josh Triplett <josh@joshtriplett.org> > > > --- > > kernel/rcutree.c | 10 ++++++++++ > > 1 files changed, 10 insertions(+), 0 deletions(-) > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > index 9854a00..5f8c4dd 100644 > > --- a/kernel/rcutree.c > > +++ b/kernel/rcutree.c > > @@ -1503,6 +1503,9 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp) > > WARN_ONCE(rdp->qlen != 0 || rdp->nxtlist != NULL, > > "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, nxtlist=%p\n", > > cpu, rdp->qlen, rdp->nxtlist); > > + init_callback_list(rdp); > > + /* Disallow further callbacks on this CPU. */ > > + rdp->nxttail[RCU_NEXT_TAIL] = NULL; > > } > > > > #else /* #ifdef CONFIG_HOTPLUG_CPU */ > > @@ -1925,6 +1928,12 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu), > > rdp = this_cpu_ptr(rsp->rda); > > > > /* Add the callback to our list. */ > > + if (unlikely(rdp->nxttail[RCU_NEXT_TAIL] == NULL)) { > > + /* _call_rcu() is illegal on offline CPU; leak the callback. */ > > + WARN_ON_ONCE(1); > > You can write this as: > > if (WARN_ON_ONCE(rdp->nxttail[RCU_NEXT_TAIL] == NULL)) > > WARN_ON_ONCE also has a built-in unlikely() already.
Indeed, I missed the fact that WARN_ON_ONCE() returns its argument. I believe that there are a number of other places in RCU where I could bury a WARN_ON_ONCE(1) into the preceding "if" statement, so added a todo-list item to check all of them and convert as appropriate.
Thanx, Paul
> > + local_irq_restore(flags); > > + return; > > + } > > ACCESS_ONCE(rdp->qlen)++; > > if (lazy) > > rdp->qlen_lazy++; > > @@ -2462,6 +2471,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible) > > rdp->qlen_last_fqs_check = 0; > > rdp->n_force_qs_snap = rsp->n_force_qs; > > rdp->blimit = blimit; > > + init_callback_list(rdp); /* Re-enable callbacks on this CPU. */ > > rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE; > > atomic_set(&rdp->dynticks->dynticks, > > (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1); > > -- > > 1.7.8 > > >
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