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authorPaul E. McKenney <paul.mckenney@linaro.org>2011-10-03 11:38:52 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2011-12-11 10:31:27 -0800
commit34240697d619c439c55f21989680024dcb604aab (patch)
tree5fa80ddb82706121520677e8e700e5081f5edaa1 /kernel/rcutree.c
parent2c01531f08f8ba663a20d8472a3032f6df133b6e (diff)
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>
Diffstat (limited to 'kernel/rcutree.c')
-rw-r--r--kernel/rcutree.c10
1 files changed, 7 insertions, 3 deletions
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1c40326724f6..69b6cdd4f944 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 */