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    SubjectRe: rcu: BUG: spinlock recursion on CPU#3, trinity-child19/5970
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    On Fri, 2012-06-29 at 10:23 -0700, Paul E. McKenney wrote:
    > On Fri, Jun 29, 2012 at 12:09:44PM +0200, Sasha Levin wrote:
    > > Hi Paul,
    > >
    > > I think I've stumbled on another bug that will increase your paranoia levels even further.
    > >
    > > I got the following lockup when fuzzing with trinity inside a KVM tools guest, using latest linux-next.
    > >
    > > It appears that it was caused by a03d6178 ("rcu: Move RCU grace-period cleanup into kthread"). This issue doesn't reproduce easily though, it took some fuzzing before hitting it.
    >
    > Hmmm... If the preemption at that point in __rcu_read_unlock() is
    > required to make this happen, then it would be pretty hard to hit.
    > I suspect that you can make it reproduce more quickly by putting
    > a udelay(10) or similar right after the assignment of INT_MIN to
    > t->rcu_read_lock_nesting in __rcu_read_unlock() in kernel/rcupdate.c.
    > Can this be reproduced while running with lockdep enabled?

    The good news are that it is much easier to reproduce it by adding a udelay(10) at the point you've mentioned.

    The bad news are that what you saw was a lockdep enabled run (CONFIG_PROVE_RCU is enabled, and lockdep was enabled). There were no lockdep warnings at any point while reproducing it.

    > Please see below for an untested patch that gets RCU out of this loop,
    > but it is quite possible that something else is involved here, so it would
    > be very good to get a lockdep run, if possible. My concern stems from the
    > fact that interrupts were enabled during the call to __rcu_read_unlock()
    > -- otherwise it would not have been preempted -- so the runqueue locks
    > were presumably not held on entry to __rcu_read_unlock().

    I've scheduled to try this patch tomorrow after a sleep, unless you preempt me first :)

    > Thanx, Paul
    >
    > ------------------------------------------------------------------------
    >
    > rcu: Avoid grace-period kthread wakeups from unsafe environments
    >
    > A soft lockup involving the run-queue locks involved the wakeup of
    > the grace-period kthread. This commit therefore defers the wakeup
    > to softirq context.
    >
    > Reported-by: Sasha Levin <levinsasha928@gmail.com>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    >
    > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > index c0e0454..cd0076e 100644
    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    > @@ -1262,6 +1262,15 @@ static int rcu_gp_kthread(void *arg)
    > }
    >
    > /*
    > + * Signal the RCU core to wake up the grace-period kthread, as we might
    > + * not be in a context where it is safe to do the wakeup directly.
    > + */
    > +static void rcu_wake_gp_kthread(struct rcu_state *rsp)
    > +{
    > + ACCESS_ONCE(rsp->wake_gp_kthread) = 1;
    > +}
    > +
    > +/*
    > * Start a new RCU grace period if warranted, re-initializing the hierarchy
    > * in preparation for detecting the next grace period. The caller must hold
    > * the root node's ->lock, which is released before return. Hard irqs must
    > @@ -1291,7 +1300,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
    >
    > rsp->gp_flags = 1;
    > raw_spin_unlock_irqrestore(&rnp->lock, flags);
    > - wake_up(&rsp->gp_wq);
    > + rcu_wake_gp_kthread(rsp);
    > }
    >
    > /*
    > @@ -1306,7 +1315,7 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
    > {
    > WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
    > raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
    > - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
    > + rcu_wake_gp_kthread(rsp); /* smp_mb() implied by wakeup. */
    > }
    >
    > /*
    > @@ -1889,6 +1898,12 @@ __rcu_process_callbacks(struct rcu_state *rsp)
    >
    > WARN_ON_ONCE(rdp->beenonline == 0);
    >
    > + /* Awaken the grace-period kthread if needed. */
    > + if (ACCESS_ONCE(rsp->wake_gp_kthread)) {
    > + if (xchg(&rsp->wake_gp_kthread, 0)) /* Prevent wakeup storms. */
    > + wake_up(&rsp->gp_wq);
    > + }
    > +
    > /*
    > * Advance callbacks in response to end of earlier grace
    > * period that some other CPU ended.
    > @@ -2304,6 +2319,12 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
    > return 1;
    > }
    >
    > + /* Does the grace-period kthread need to be awakened? */
    > + if (ACCESS_ONCE(rsp->wake_gp_kthread)) {
    > + rdp->n_wake_gp_kthread++;
    > + return 1;
    > + }
    > +
    > /* nothing to do */
    > rdp->n_rp_need_nothing++;
    > return 0;
    > diff --git a/kernel/rcutree.h b/kernel/rcutree.h
    > index 280ad7c..89a190c 100644
    > --- a/kernel/rcutree.h
    > +++ b/kernel/rcutree.h
    > @@ -313,6 +313,7 @@ struct rcu_data {
    > unsigned long n_rp_cpu_needs_gp;
    > unsigned long n_rp_gp_completed;
    > unsigned long n_rp_gp_started;
    > + unsigned long n_wake_gp_kthread;
    > unsigned long n_rp_need_nothing;
    >
    > /* 6) _rcu_barrier() callback. */
    > @@ -381,6 +382,7 @@ struct rcu_state {
    > struct task_struct *gp_kthread; /* Task for grace periods. */
    > wait_queue_head_t gp_wq; /* Where GP task waits. */
    > int gp_flags; /* Commands for GP task. */
    > + int wake_gp_kthread; /* Awaken GP task? */
    >
    > /* End of fields guarded by root rcu_node's lock. */
    >
    >
    > --
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