From: Jens Axboe <axboe@kernel.dk>
To: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: [PATCH] task_work: only grab task signal lock when needed
Date: Thu, 13 Aug 2020 09:07:13 -0600 [thread overview]
Message-ID: <90d18cac-019e-eb49-8b33-32066d4e24aa@kernel.dk> (raw)
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 6f16f7c5d375..42b67d2cea37 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2541,7 +2541,21 @@ bool get_signal(struct ksignal *ksig)
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 5c0848ca1287..613b2d634af8 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -42,7 +42,13 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify)
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);
--
Jens Axboe
next reply other threads:[~2020-08-13 15:07 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-13 15:07 Jens Axboe [this message]
-- strict thread matches above, loose matches on Subject: below --
2020-08-11 14:25 [PATCH] task_work: only grab task signal lock when needed Jens Axboe
2020-08-11 15:23 ` Oleg Nesterov
2020-08-11 16:45 ` Jens Axboe
2020-08-12 14:54 ` Oleg Nesterov
2020-08-12 20:06 ` Peter Zijlstra
2020-08-12 23:13 ` Jens Axboe
2020-08-13 11:48 ` Oleg Nesterov
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