rcu: Grace-period initialization excludes only RCU notifier

Kirill noted the following deadlock cycle on shutdown involving padata:

> With commit 755609a9087fa983f567dc5452b2fa7b089b591f I've got deadlock on
> poweroff.
>
> It guess it happens because of race for cpu_hotplug.lock:
>
>       CPU A                                   CPU B
> disable_nonboot_cpus()
> _cpu_down()
> cpu_hotplug_begin()
>  mutex_lock(&cpu_hotplug.lock);
> __cpu_notify()
> padata_cpu_callback()
> __padata_remove_cpu()
> padata_replace()
> synchronize_rcu()
>                                       rcu_gp_kthread()
>                                       get_online_cpus();
>                                       mutex_lock(&cpu_hotplug.lock);

It would of course be good to eliminate grace-period delays from
CPU-hotplug notifiers, but that is a separate issue.  Deadlock is
not an appropriate diagnostic for excessive CPU-hotplug latency.

Fortunately, grace-period initialization does not actually need to
exclude all of the CPU-hotplug operation, but rather only RCU's own
CPU_UP_PREPARE and CPU_DEAD CPU-hotplug notifiers.  This commit therefore
introduces a new per-rcu_state onoff_mutex that provides the required
concurrency control in place of the get_online_cpus() that was previously
in rcu_gp_init().

Reported-by: "Kirill A. Shutemov" <kirill@shutemov.name>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Kirill A. Shutemov <kirill@shutemov.name>

1 files changed, 6 insertions, 0 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 5faf05d68326..a240f032848e 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -394,11 +394,17 @@ struct rcu_state {
         struct rcu_head **orphan_donetail;      /* Tail of above. */
         long qlen_lazy;                         /* Number of lazy callbacks. */
         long qlen;                              /* Total number of callbacks. */
+        /* End of fields guarded by onofflock. */
+
+        struct mutex onoff_mutex;               /* Coordinate hotplug & GPs. */
+
         struct mutex barrier_mutex;             /* Guards barrier fields. */
         atomic_t barrier_cpu_count;             /* # CPUs waiting on. */
         struct completion barrier_completion;   /* Wake at barrier end. */
         unsigned long n_barrier_done;           /* ++ at start and end of */
                                                 /*  _rcu_barrier(). */
+        /* End of fields guarded by barrier_mutex. */
+
         unsigned long jiffies_force_qs;         /* Time at which to invoke */
                                                 /*  force_quiescent_state(). */
         unsigned long n_force_qs;               /* Number of calls to */