[PATCH tip/core/rcu 14/30] refperf: Convert reader_task structure's "start" field to int

paulmck
2020-06-23T00:30:42Z

This commit converts the reader_task structure's "start" field to int
in order to demote a full barrier to an smp_load_acquire() and also to
simplify the code a bit. While in the area, and to enlist the compiler's
help in ensuring that nothing was missed, the field's name was changed
to start_reader.

Also while in the area, change the main_func() store to use
smp_store_release() to further fortify against wait/wake races.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/refperf.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/refperf.c b/kernel/rcu/refperf.c
index 8815ccf..2fd3ed1 100644
--- a/kernel/rcu/refperf.c
+++ b/kernel/rcu/refperf.c
@@ -80,7 +80,7 @@ torture_param(bool, shutdown, REFPERF_SHUTDOWN,

struct reader_task {
struct task_struct *task;
- atomic_t start;
+ int start_reader;
wait_queue_head_t wq;
u64 last_duration_ns;
};
@@ -243,7 +243,7 @@ ref_perf_reader(void *arg)
VERBOSE_PERFOUT("ref_perf_reader %ld: waiting to start next experiment on cpu %d", me, smp_processor_id());

// Wait for signal that this reader can start.
- wait_event(rt->wq, (atomic_read(&nreaders_exp) && atomic_read(&rt->start)) ||
+ wait_event(rt->wq, (atomic_read(&nreaders_exp) && smp_load_acquire(&rt->start_reader)) ||
torture_must_stop());

if (torture_must_stop())
@@ -252,8 +252,7 @@ ref_perf_reader(void *arg)
// Make sure that the CPU is affinitized appropriately during testing.
WARN_ON_ONCE(smp_processor_id() != me);

- smp_mb__before_atomic();
- atomic_dec(&rt->start);
+ WRITE_ONCE(rt->start_reader, 0);

VERBOSE_PERFOUT("ref_perf_reader %ld: experiment %d started", me, exp_idx);

@@ -372,7 +371,7 @@ static int main_func(void *arg)
exp_idx = exp;

for (r = 0; r < nreaders; r++) {
- atomic_set(&reader_tasks[r].start, 1);
+ smp_store_release(&reader_tasks[r].start_reader, 1);
wake_up(&reader_tasks[r].wq);
}

--
2.9.5