task_work: only grab task signal lock when needed
If JOBCTL_TASK_WORK is already set on the targeted task, then we need
not go through {lock,unlock}_task_sighand() to set it again and queue
a signal wakeup. This is safe as we're checking it _after_ adding the
new task_work with cmpxchg().
The ordering is as follows:
task_work_add() get_signal()
--------------------------------------------------------------
STORE(task->task_works, new_work); STORE(task->jobctl);
mb(); mb();
LOAD(task->jobctl); LOAD(task->task_works);
This speeds up TWA_SIGNAL handling quite a bit, which is important now
that io_uring is relying on it for all task_work deliveries.
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/kernel/signal.c b/kernel/signal.c
index 6f16f7c..42b67d2 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2541,7 +2541,21 @@
relock:
spin_lock_irq(&sighand->siglock);
- current->jobctl &= ~JOBCTL_TASK_WORK;
+ /*
+ * Make sure we can safely read ->jobctl() in task_work add. As Oleg
+ * states:
+ *
+ * It pairs with mb (implied by cmpxchg) before READ_ONCE. So we
+ * roughly have
+ *
+ * task_work_add: get_signal:
+ * STORE(task->task_works, new_work); STORE(task->jobctl);
+ * mb(); mb();
+ * LOAD(task->jobctl); LOAD(task->task_works);
+ *
+ * and we can rely on STORE-MB-LOAD [ in task_work_add].
+ */
+ smp_store_mb(current->jobctl, current->jobctl & ~JOBCTL_TASK_WORK);
if (unlikely(current->task_works)) {
spin_unlock_irq(&sighand->siglock);
task_work_run();
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 5c0848c..613b2d6 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -42,7 +42,13 @@
set_notify_resume(task);
break;
case TWA_SIGNAL:
- if (lock_task_sighand(task, &flags)) {
+ /*
+ * Only grab the sighand lock if we don't already have some
+ * task_work pending. This pairs with the smp_store_mb()
+ * in get_signal(), see comment there.
+ */
+ if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
+ lock_task_sighand(task, &flags)) {
task->jobctl |= JOBCTL_TASK_WORK;
signal_wake_up(task, 0);
unlock_task_sighand(task, &flags);