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Commit 95f0c1de authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Disable preemption in rcu_blocking_is_gp() 



It is time to optimize CONFIG_TREE_PREEMPT_RCU's synchronize_rcu()
for uniprocessor optimization, which means that rcu_blocking_is_gp()
can no longer rely on RCU read-side critical sections having disabled
preemption.  This commit therefore disables preemption across
rcu_blocking_is_gp()'s scan of the cpu_online_mask.

(Updated from previous version to fix embarrassing bug spotted by
Wu Fengguang.)

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 1c17e4d4

kernel/rcutree.c

+6 −18
Original line number Diff line number Diff line
@@ -1981,28 +1981,16 @@ EXPORT_SYMBOL_GPL(call_rcu_bh); 
 * occasionally incorrectly indicate that there are multiple CPUs online

 * when there was in fact only one the whole time, as this just adds

 * some overhead: RCU still operates correctly.

 *

 * Of course, sampling num_online_cpus() with preemption enabled can

 * give erroneous results if there are concurrent CPU-hotplug operations.

 * For example, given a demonic sequence of preemptions in num_online_cpus()

 * and CPU-hotplug operations, there could be two or more CPUs online at

 * all times, but num_online_cpus() might well return one (or even zero).

 *

 * However, all such demonic sequences require at least one CPU-offline

 * operation.  Furthermore, rcu_blocking_is_gp() giving the wrong answer

 * is only a problem if there is an RCU read-side critical section executing

 * throughout.  But RCU-sched and RCU-bh read-side critical sections

 * disable either preemption or bh, which prevents a CPU from going offline.

 * Therefore, the only way that rcu_blocking_is_gp() can incorrectly return

 * that there is only one CPU when in fact there was more than one throughout

 * is when there were no RCU readers in the system.  If there are no

 * RCU readers, the grace period by definition can be of zero length,

 * regardless of the number of online CPUs.

 */

static inline int rcu_blocking_is_gp(void)

{

	int ret;



	might_sleep();  /* Check for RCU read-side critical section. */

	return num_online_cpus() <= 1;

	preempt_disable();

	ret = num_online_cpus() <= 1;

	preempt_enable();

	return ret;

}



/**