rcu: Disable preemption in rcu_is_cpu_idle()
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>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1c40326..69b6cdd 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -556,12 +556,16 @@
* 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 */
