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rcu: Grace-period initialization excludes only RCU notifier
Kirill noted the following deadlock cycle on shutdown involving padata: > With commit 755609a 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" <firstname.lastname@example.org> Signed-off-by: Paul E. McKenney <email@example.com> Tested-by: Kirill A. Shutemov <firstname.lastname@example.org>
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