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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-08-25 20:25:06 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-09-18 16:22:27 -0700
commitdd56af42bd829c6e770ed69812bd65a04eaeb1e4 (patch)
tree6d510ac693c3dfb05dcc383cb60acd3e1e8cab6c /kernel/cpu.c
parentec4518aad8329364af373f4bf7f4eff25a01a339 (diff)
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 <paulmck@linux.vnet.ibm.com> Cc: Josh Triplett <josh@joshtriplett.org> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net> Tested-by: Lan Tianyu <tianyu.lan@intel.com>
Diffstat (limited to 'kernel/cpu.c')
-rw-r--r--kernel/cpu.c16
1 files changed, 15 insertions, 1 deletions
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 81e2a388a0f6..356450f09c1f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -79,6 +79,8 @@ static struct {
/* Lockdep annotations for get/put_online_cpus() and cpu_hotplug_begin/end() */
#define cpuhp_lock_acquire_read() lock_map_acquire_read(&cpu_hotplug.dep_map)
+#define cpuhp_lock_acquire_tryread() \
+ lock_map_acquire_tryread(&cpu_hotplug.dep_map)
#define cpuhp_lock_acquire() lock_map_acquire(&cpu_hotplug.dep_map)
#define cpuhp_lock_release() lock_map_release(&cpu_hotplug.dep_map)
@@ -91,10 +93,22 @@ void get_online_cpus(void)
mutex_lock(&cpu_hotplug.lock);
cpu_hotplug.refcount++;
mutex_unlock(&cpu_hotplug.lock);
-
}
EXPORT_SYMBOL_GPL(get_online_cpus);
+bool try_get_online_cpus(void)
+{
+ if (cpu_hotplug.active_writer == current)
+ return true;
+ if (!mutex_trylock(&cpu_hotplug.lock))
+ return false;
+ cpuhp_lock_acquire_tryread();
+ cpu_hotplug.refcount++;
+ mutex_unlock(&cpu_hotplug.lock);
+ return true;
+}
+EXPORT_SYMBOL_GPL(try_get_online_cpus);
+
void put_online_cpus(void)
{
if (cpu_hotplug.active_writer == current)

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