Commit-ID: 34240697d619c439c55f21989680024dcb604aab Gitweb: http://git.kernel.org/tip/34240697d619c439c55f21989680024dcb604aab Author: Paul E. McKenney <paul.mckenney@xxxxxxxxxx> AuthorDate: Mon, 3 Oct 2011 11:38:52 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Sun, 11 Dec 2011 10:31:27 -0800 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@xxxxxxxxxxxxxxxxxx> Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx> --- kernel/rcutree.c | 10 +++++++--- 1 files changed, 7 insertions(+), 3 deletions(-) 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 @@ 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 */ -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html