The following commit has been merged into the core/rcu branch of tip:

Commit-ID: 127e29815b4b2206c0a97ac1d83f92ffc0e25c34 Gitweb: https://git.kernel.org/tip/127e29815b4b2206c0a97ac1d83f92ffc0e25c34 Author: Paul E. McKenney paulmck@kernel.org AuthorDate: Tue, 11 Feb 2020 06:17:33 -08:00 Committer: Paul E. McKenney paulmck@kernel.org CommitterDate: Sat, 21 Mar 2020 16:14:25 -07:00

rcu: Make rcu_barrier() account for offline no-CBs CPUs

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 [ paulmck: Apply optimization feedback from Boqun Feng. ] Cc: stable@vger.kernel.org # 5.5.x --- include/trace/events/rcu.h | 1 + kernel/rcu/tree.c | 36 ++++++++++++++++++++++++------------ 2 files changed, 25 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 739788f..35292c8 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3097,9 +3097,10 @@ 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; @@ -3126,7 +3127,7 @@ 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);

@@ -3149,13 +3150,14 @@ 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. */ init_completion(&rcu_state.barrier_completion); - atomic_set(&rcu_state.barrier_cpu_count, 1); + atomic_set(&rcu_state.barrier_cpu_count, 2); get_online_cpus();

/* @@ -3165,13 +3167,23 @@ 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) && !rcu_segcblist_is_offloaded(&rdp->cblist)) continue; - 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 (rcu_segcblist_n_cbs(&rdp->cblist) && + cpu_is_offline(cpu)) { + rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu, + rcu_state.barrier_sequence); + local_irq_disable(); + rcu_barrier_func((void *)cpu); + local_irq_enable(); + } else if (cpu_is_offline(cpu)) { + rcu_barrier_trace(TPS("OfflineNoCBNoQ"), cpu, + rcu_state.barrier_sequence); } else { rcu_barrier_trace(TPS("OnlineNQ"), cpu, rcu_state.barrier_sequence); @@ -3183,7 +3195,7 @@ void rcu_barrier(void) * Now that we have an rcu_barrier_callback() callback on each * CPU, and thus each counted, remove the initial count. */ - if (atomic_dec_and_test(&rcu_state.barrier_cpu_count)) + if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count)) complete(&rcu_state.barrier_completion);

/* Wait for all rcu_barrier_callback() callbacks to be invoked. */