On Tue, Feb 25, 2020 at 07:14:55PM -0800, Paul E. McKenney wrote:
On Tue, Feb 25, 2020 at 06:24:36PM +0800, Boqun Feng wrote:
Hi Paul,
On Fri, Feb 14, 2020 at 03:56:07PM -0800, paulmck@kernel.org wrote:
From: "Paul E. McKenney" paulmck@kernel.org
Currently, rcu_barrier() ignores offline CPUs, However, it is possible for an offline no-CBs CPU to have callbacks queued, and rcu_barrier() must wait for those callbacks. This commit therefore makes rcu_barrier() directly invoke the rcu_barrier_func() with interrupts disabled for such CPUs. This requires passing the CPU number into this function so that it can entrain the rcu_barrier() callback onto the correct CPU's callback list, given that the code must instead execute on the current CPU.
While in the area, this commit fixes a bug where the first CPU's callback might have been invoked before rcu_segcblist_entrain() returned, which would also result in an early wakeup.
Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs") Signed-off-by: Paul E. McKenney paulmck@kernel.org Cc: stable@vger.kernel.org # 5.5.x
include/trace/events/rcu.h | 1 + kernel/rcu/tree.c | 32 ++++++++++++++++++++------------ 2 files changed, 21 insertions(+), 12 deletions(-)
diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h index 5e49b06..d56d54c 100644 --- a/include/trace/events/rcu.h +++ b/include/trace/events/rcu.h @@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
- "Begin": rcu_barrier() started.
- "EarlyExit": rcu_barrier() piggybacked, thus early exit.
- "Inc1": rcu_barrier() piggyback check counter incremented.
- "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
- "OnlineQ": rcu_barrier() found online CPU with callbacks.
- "OnlineNQ": rcu_barrier() found online CPU, no callbacks.
- "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index d15041f..160643e 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp) /*
- Called with preemption disabled, and from cross-cpu IRQ context.
*/ -static void rcu_barrier_func(void *unused) +static void rcu_barrier_func(void *cpu_in) {
- struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
- uintptr_t cpu = (uintptr_t)cpu_in;
- struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence); rdp->barrier_head.func = rcu_barrier_callback; @@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused) */ void rcu_barrier(void) {
- int cpu;
- uintptr_t cpu; struct rcu_data *rdp; unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
@@ -3150,13 +3151,14 @@ void rcu_barrier(void) rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence); /*
* 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.
* Initialize the count to two rather than to zero in order* to avoid a too-soon return to zero in case of an immediate* invocation of the just-enqueued callback (or preemption of* this task). Exclude CPU-hotplug operations to ensure that no* offline non-offloaded CPU has callbacks queued.
- atomic_set(&rcu_state.barrier_cpu_count, 1);
- atomic_set(&rcu_state.barrier_cpu_count, 2); get_online_cpus();
/* @@ -3166,13 +3168,19 @@ void rcu_barrier(void) */ for_each_possible_cpu(cpu) { rdp = per_cpu_ptr(&rcu_data, cpu);
if (!cpu_online(cpu) &&
if (cpu_is_offline(cpu) &&
if (rcu_segcblist_n_cbs(&rdp->cblist)) {
if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) { rcu_barrier_trace(TPS("OnlineQ"), cpu, rcu_state.barrier_sequence);
smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);} else if (cpu_is_offline(cpu)) {I wonder whether this should be:
else if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_is_offline(cpu))? Because I think we only want to queue the barrier call back if there are callbacks for a particular CPU. Am I missing something subtle?
I don't believe that you are missing anything at all!
Thank you very much -- this bug would not have shown up in any validation setup that I am aware of. ;-)
And with additional adjustment to make tracing accurate.
Thanx, Paul