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Subject[PATCH 4.18 207/235] rcu: Fix grace-period hangs due to race with CPU offline
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4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

[ Upstream commit 1e64b15a4b102e1cd059d4d798b7a78f93341333 ]

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
the 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 <paulmck@linux.vnet.ibm.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/rcu/tree.c | 6 ++++++
kernel/rcu/tree.h | 4 ++++
2 files changed, 10 insertions(+)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -102,6 +102,7 @@ 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);
@@ -1925,11 +1926,13 @@ static bool rcu_gp_init(struct rcu_state
*/
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;
}

@@ -1964,6 +1967,7 @@ static bool rcu_gp_init(struct rcu_state
}

raw_spin_unlock_irq_rcu_node(rnp);
+ spin_unlock(&rsp->ofl_lock);
}

/*
@@ -3725,9 +3729,11 @@ static void rcu_cleanup_dying_idle_cpu(i

/* 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. */
rnp->qsmaskinitnext &= ~mask;
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ spin_unlock(&rsp->ofl_lock);
}

/*
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -384,6 +384,10 @@ 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. */

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