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SubjectRe: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop_machine()")
On Tue, Oct 02, 2012 at 06:14:08PM +0200, Jiri Kosina wrote:
> Hi,
>
> this commit:
>
> ==
> 1331e7a1bbe1f11b19c4327ba0853bee2a606543 is the first bad commit
> commit 1331e7a1bbe1f11b19c4327ba0853bee2a606543
> Author: Paul E. McKenney <paul.mckenney@linaro.org>
> Date: Thu Aug 2 17:43:50 2012 -0700
>
> rcu: Remove _rcu_barrier() dependency on __stop_machine()
>
> Currently, _rcu_barrier() relies on preempt_disable() to prevent
> any CPU from going offline, which in turn depends on CPU hotplug's
> use of __stop_machine().
>
> This patch therefore makes _rcu_barrier() use get_online_cpus() to
> block CPU-hotplug operations. This has the added benefit of removing
> the need for _rcu_barrier() to adopt callbacks: Because CPU-hotplug
> operations are excluded, there can be no callbacks to adopt. This
> commit simplifies the code accordingly.
>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> ==
>
> is causing lockdep to complain (see the full trace below). I haven't yet
> had time to analyze what exactly is happening, and probably will not have
> time to do so until tomorrow, so just sending this as a heads-up in case
> anyone sees the culprit immediately.

Hmmm... Does the following patch help? It swaps the order in which
rcu_barrier() acquires the hotplug and rcu_barrier locks.

Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 2ad9e81..2c71d61 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2560,6 +2560,9 @@ static void _rcu_barrier(struct rcu_state *rsp)

_rcu_barrier_trace(rsp, "Begin", -1, snap);

+ /* Exclude CPU-hotplug, ensuring that no offline CPU has callbacks. */
+ get_online_cpus();
+
/* Take mutex to serialize concurrent rcu_barrier() requests. */
mutex_lock(&rsp->barrier_mutex);

@@ -2581,6 +2584,7 @@ static void _rcu_barrier(struct rcu_state *rsp)
_rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);
smp_mb(); /* caller's subsequent code after above check. */
mutex_unlock(&rsp->barrier_mutex);
+ put_online_cpus();
return;
}

@@ -2597,8 +2601,7 @@ static void _rcu_barrier(struct rcu_state *rsp)
/*
* Initialize the count to one rather than to zero in order to
* avoid a too-soon return to zero in case of a short grace period
- * (or preemption of this task). Exclude CPU-hotplug operations
- * to ensure that no offline CPU has callbacks queued.
+ * (or preemption of this task).
*/
init_completion(&rsp->barrier_completion);
atomic_set(&rsp->barrier_cpu_count, 1);
@@ -2620,7 +2623,6 @@ static void _rcu_barrier(struct rcu_state *rsp)
rsp->n_barrier_done);
}
}
- put_online_cpus();

/*
* Now that we have an rcu_barrier_callback() callback on each
@@ -2639,8 +2641,9 @@ static void _rcu_barrier(struct rcu_state *rsp)
/* Wait for all rcu_barrier_callback() callbacks to be invoked. */
wait_for_completion(&rsp->barrier_completion);

- /* Other rcu_barrier() invocations can now safely proceed. */
+ /* Hotplug and rcu_barrier() invocations can now safely proceed. */
mutex_unlock(&rsp->barrier_mutex);
+ put_online_cpus();
}

/**


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