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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: 5d6742b377 ("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
This commit is contained in:
Paul E. McKenney 2020-02-11 06:17:33 -08:00
parent 0f11ad323d
commit 127e29815b
2 changed files with 25 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/trace/events/rcu.h
View File

@ -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.

36
kernel/rcu/tree.c
View File

@ -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. */