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rcu: Eliminate deadlock between CPU hotplug and expedited grace periods

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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>
This commit is contained in:
Paul E. McKenney 2014-08-25 20:25:06 -07:00
parent ec4518aad8
commit dd56af42bd
5 changed files with 35 additions and 14 deletions
Download patch file Download diff file Expand all files Collapse all files

16
kernel/cpu.c
View file

@ -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)