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Subject[PATCH RFC tip/core/rcu 10/28] rcu: Disable preemption in rcu_is_cpu_idle()
Date
From: Paul E. McKenney <paul.mckenney@linaro.org>

Because rcu_is_cpu_idle() is to be used to check for extended quiescent
states in RCU-preempt read-side critical sections, it cannot assume that
preemption is disabled. And preemption must be disabled when accessing
the dyntick-idle state, because otherwise the following sequence of events
could occur:

1. Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the pointer
to CPU 1's per-CPU variables.

2. Task B preempts Task A and starts running on CPU 1.

3. Task A migrates to CPU 2.

4. Task B blocks, leaving CPU 1 idle.

5. Task A continues execution on CPU 2, accessing CPU 1's dyntick-idle
information using the pointer fetched in step 1 above, and finds
that CPU 1 is idle.

6. Task A therefore incorrectly concludes that it is executing in
an extended quiescent state, possibly issuing a spurious splat.

Therefore, this commit disables preemption within the rcu_is_cpu_idle()
function.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 10 +++++++---
1 files changed, 7 insertions(+), 3 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index abb5167..c097394 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -556,12 +556,16 @@ void rcu_nmi_exit(void)
* rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle
*
* If the current CPU is in its idle loop and is neither in an interrupt
- * or NMI handler, return true. The caller must have at least disabled
- * preemption.
+ * or NMI handler, return true.
*/
int rcu_is_cpu_idle(void)
{
- return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
+ int ret;
+
+ preempt_disable();
+ ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
+ preempt_enable();
+ return ret;
}

#endif /* #ifdef CONFIG_PROVE_RCU */
--
1.7.3.2


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