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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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2012-06-19 11:58:27 -07:00
parent 1c17e4d443
commit 95f0c1de3e
1 changed files with 6 additions and 18 deletions
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24
kernel/rcutree.c
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@ -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;
}
/**