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From: Lai Jiangshan on 28 Mar 2010 22:50

Even though in user mode or idle mode, rcu_check_callbacks() is not
context switch, so we don't call rcu_preempt_note_context_switch()
in rcu_check_callbacks().

Though there is no harm that calls rcu_preempt_note_context_switch()
in rcu_check_callbacks(), but it is waste.

rcu_check_callbacks()
rcu_sched_qs()
rcu_preempt_note_context_switch()
Now, ->rcu_read_lock_nesting == 0, so we just calls
rcu_preempt_qs(), but, rcu_preempt_check_callbacks()
will call it again and set the ->rcu_read_unlock_special
correct again.

So let rcu_preempt_check_callbacks() handle things for us.

Signed-off-by: Lai Jiangshan <laijs(a)cn.fujitsu.com>
---
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 3ec8160..c7847ba 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -95,7 +95,7 @@ static int rcu_gp_in_progress(struct rcu_state *rsp)
* how many quiescent states passed, just if there was at least
* one since the start of the grace period, this just sets a flag.
*/
-void rcu_sched_qs(int cpu)
+static void __rcu_sched_qs(int cpu)
{
struct rcu_data *rdp;

@@ -103,6 +103,11 @@ void rcu_sched_qs(int cpu)
rdp->passed_quiesc_completed = rdp->gpnum - 1;
barrier();
rdp->passed_quiesc = 1;
+}
+
+void rcu_sched_qs(int cpu)
+{
+ __rcu_sched_qs(cpu);
rcu_preempt_note_context_switch(cpu);
}

@@ -1138,12 +1143,12 @@ void rcu_check_callbacks(int cpu, int user)
* a quiescent state, so note it.
*
* No memory barrier is required here because both
- * rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
+ * __rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
* variables that other CPUs neither access nor modify,
* at least not while the corresponding CPU is online.
*/

- rcu_sched_qs(cpu);
+ __rcu_sched_qs(cpu);
rcu_bh_qs(cpu);

} else if (!in_softirq()) {

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Prev: [PATCH 25/31] x86, lmb: Use lmb_memory_size()/lmb_free_memory_size() to get correct dma_reserve
Next: [PATCH 03/31] x86: Do not free zero sized per cpu areas