On Tue, Jun 26, 2018 at 11:29:50AM -0700, Paul E. McKenney wrote:
> On Tue, Jun 26, 2018 at 07:51:19PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 26, 2018 at 10:10:39AM -0700, Paul E. McKenney wrote:
> > > Without special fail-safe quiescent-state-propagation checks, grace-period
> > > hangs can result from the following scenario:
> > >
> > > 1. CPU 1 goes offline.
> > >
> > > 2. Because CPU 1 is the only CPU in the system blocking the current
> > > grace period, as soon as rcu_cleanup_dying_idle_cpu()'s call to
> > > rcu_report_qs_rnp() returns.
> > >
> > > 3. At this point, the leaf rcu_node structure's ->lock is no longer
> > > held: rcu_report_qs_rnp() has released it, as it must in order
> > > to awaken the RCU grace-period kthread.
> > >
> > > 4. At this point, that same leaf rcu_node structure's
> > > ->qsmaskinitnext
> > > field still records CPU 1 as being online. This is absolutely
> > > necessary because the scheduler uses RCU, and ->qsmaskinitnext
> >
> > Can you expand a bit on this, where does the scheduler care about the
> > online state of the CPU that's about to call into arch_cpu_idle_dead()?
>
> Because the CPU does a context switch between the time that the CPU gets
> marked offline from the viewpoint of cpu_offline() and the time that
> the CPU finally makes it to arch_cpu_idle_dead(). Plus reporting the
> quiescent state (rcu_report_qs_rnp()) can result in waking up RCU's
> grace-period kthread. During that context switch and that wakeup,
> the scheduler needs RCU to continue paying attention to the outgoing
> CPU, right?
And is the following a reasonable expansion?
Thanx, Paul
------------------------------------------------------------------------
commit 2e5b2ff4047b138d6b56e4e3ba91bc47503cdebe
Author: Paul E. McKenney <[email protected]>
Date: Fri May 25 19:23:09 2018 -0700
rcu: Fix grace-period hangs due to race with CPU offline
Without special fail-safe quiescent-state-propagation checks, grace-period
hangs can result from the following scenario:
1. CPU 1 goes offline.
2. Because CPU 1 is the only CPU in the system blocking the current
grace period, the grace period ends as soon as
rcu_cleanup_dying_idle_cpu()'s call to rcu_report_qs_rnp()
returns.
3. At this point, the leaf rcu_node structure's ->lock is no longer
held: rcu_report_qs_rnp() has released it, as it must in order
to awaken the RCU grace-period kthread.
4. At this point, that same leaf rcu_node structure's ->qsmaskinitnext
field still records CPU 1 as being online. This is absolutely
necessary because the scheduler uses RCU (in this case on the
wake-up path while awakening RCU's grace-period kthread), and
->qsmaskinitnext contains RCU's idea as to which CPUs are online.
Therefore, invoking rcu_report_qs_rnp() after clearing CPU 1's
bit from ->qsmaskinitnext would result in a lockdep-RCU splat
due to RCU being used from an offline CPU.
5. RCU's grace-period kthread awakens, sees that the old grace period
has completed and that a new one is needed. It therefore starts
a new grace period, but because CPU 1's leaf rcu_node structure's
->qsmaskinitnext field still shows CPU 1 as being online, this new
grace period is initialized to wait for a quiescent state from the
now-offline CPU 1.
6. Without the fail-safe force-quiescent-state checks, there would
be no quiescent state from the now-offline CPU 1, which would
eventually result in RCU CPU stall warnings and memory exhaustion.
It would be good to get rid of the special fail-safe quiescent-state
propagation checks, and thus it would be good to fix things so that
he above scenario cannot happen. This commit therefore adds a new
->ofl_lock to the rcu_state structure. This lock is held by rcu_gp_init()
across the applying of buffered online and offline operations to the
rcu_node tree, and it is also held by rcu_cleanup_dying_idle_cpu()
when buffering a new offline operation. This prevents rcu_gp_init()
from acquiring the leaf rcu_node structure's lock during the interval
between when rcu_cleanup_dying_idle_cpu() invokes rcu_report_qs_rnp(),
which releases ->lock and the re-acquisition of that same lock.
This in turn prevents the failure scenario outlined above, and will
hopefully eventually allow removal of the offline-CPU checks from the
force-quiescent-state code path.
Signed-off-by: Paul E. McKenney <[email protected]>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2cfd5d3da4f8..bb8f45c0fa68 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -101,6 +101,7 @@ struct rcu_state sname##_state = { \
.abbr = sabbr, \
.exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \
.exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \
+ .ofl_lock = __SPIN_LOCK_UNLOCKED(sname##_state.ofl_lock), \
}
RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
@@ -1900,11 +1901,13 @@ static bool rcu_gp_init(struct rcu_state *rsp)
*/
rcu_for_each_leaf_node(rsp, rnp) {
rcu_gp_slow(rsp, gp_preinit_delay);
+ spin_lock(&rsp->ofl_lock);
raw_spin_lock_irq_rcu_node(rnp);
if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
!rnp->wait_blkd_tasks) {
/* Nothing to do on this leaf rcu_node structure. */
raw_spin_unlock_irq_rcu_node(rnp);
+ spin_unlock(&rsp->ofl_lock);
continue;
}
@@ -1940,6 +1943,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
}
raw_spin_unlock_irq_rcu_node(rnp);
+ spin_unlock(&rsp->ofl_lock);
}
/*
@@ -3747,6 +3751,7 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct
rcu_state *rsp)
/* Remove outgoing CPU from mask in the leaf rcu_node structure. */
mask = rdp->grpmask;
+ spin_lock(&rsp->ofl_lock);
raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order
guarantee. */
if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
/* Report quiescent state -before- changing ->qsmaskinitnext! */
@@ -3755,6 +3760,7 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct
rcu_state *rsp)
}
rnp->qsmaskinitnext &= ~mask;
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ spin_unlock(&rsp->ofl_lock);
}
/*
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 3def94fc9c74..6683da6e4ecc 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -363,6 +363,10 @@ struct rcu_state {
const char *name; /* Name of structure. */
char abbr; /* Abbreviated name. */
struct list_head flavors; /* List of RCU flavors. */
+
+ spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
+ /* Synchronize offline with */
+ /* GP pre-initialization. */
};
/* Values for rcu_state structure's gp_flags field. */
