text: From: Paul E. McKenney <paul.mckenney@linaro.org>Because rcu_is_cpu_idle() is to be used to check for extended quiescentstates in RCU-preempt read-side critical sections, it cannot assume thatpreemption is disabled. And preemption must be disabled when accessingthe dyntick-idle state, because otherwise the following sequence of eventscould 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.cindex 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