From dd56af42bd829c6e770ed69812bd65a04eaeb1e4 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Mon, 25 Aug 2014 20:25:06 -0700 Subject: rcu: Eliminate deadlock between CPU hotplug and expedited grace periods Currently, the expedited grace-period primitives do get_online_cpus(). This greatly simplifies their implementation, but means that calls to them holding locks that are acquired by CPU-hotplug notifiers (to say nothing of calls to these primitives from CPU-hotplug notifiers) can deadlock. But this is starting to become inconvenient, as can be seen here: https://lkml.org/lkml/2014/8/5/754. The problem in this case is that some developers need to acquire a mutex from a CPU-hotplug notifier, but also need to hold it across a synchronize_rcu_expedited(). As noted above, this currently results in deadlock. This commit avoids the deadlock and retains the simplicity by creating a try_get_online_cpus(), which returns false if the get_online_cpus() reference count could not immediately be incremented. If a call to try_get_online_cpus() returns true, the expedited primitives operate as before. If a call returns false, the expedited primitives fall back to normal grace-period operations. This falling back of course results in increased grace-period latency, but only during times when CPU hotplug operations are actually in flight. The effect should therefore be negligible during normal operation. Signed-off-by: Paul E. McKenney Cc: Josh Triplett Cc: "Rafael J. Wysocki" Tested-by: Lan Tianyu --- include/linux/lockdep.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux/lockdep.h') diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h index 008388f920d..4f86465cc31 100644 --- a/include/linux/lockdep.h +++ b/include/linux/lockdep.h @@ -505,6 +505,7 @@ static inline void print_irqtrace_events(struct task_struct *curr) #define lock_map_acquire(l) lock_acquire_exclusive(l, 0, 0, NULL, _THIS_IP_) #define lock_map_acquire_read(l) lock_acquire_shared_recursive(l, 0, 0, NULL, _THIS_IP_) +#define lock_map_acquire_tryread(l) lock_acquire_shared_recursive(l, 0, 1, NULL, _THIS_IP_) #define lock_map_release(l) lock_release(l, 1, _THIS_IP_) #ifdef CONFIG_PROVE_LOCKING -- cgit v1.2.3-70-g09d2