From a4fbe35a124526e6759be07bd9c7ea796ba1e00d Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 7 Oct 2012 08:36:12 -0700 Subject: 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" Signed-off-by: Paul E. McKenney Tested-by: Kirill A. Shutemov --- kernel/rcutree.h | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'kernel/rcutree.h') 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 */ -- cgit v1.2.3