linux-kernel - [PATCH 3/3] task_work: use TIF

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Date:   Thu,  1 Oct 2020 13:42:08 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Cc:     peterz@...radead.org, oleg@...hat.com, tglx@...utronix.de,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 3/3] task_work: use TIF_TASKWORK if available

If the arch supports TIF_TASKWORK, then use that for TWA_SIGNAL as
it's more efficient than using the signal delivery method. This is
especially true on threaded applications, where ->sighand is shared
across threads.

Signed-off-by: Jens Axboe <axboe@...nel.dk>
---
 kernel/task_work.c | 48 ++++++++++++++++++++++++++++++++++------------
 1 file changed, 36 insertions(+), 12 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 613b2d634af8..ae317cfe86b8 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -5,6 +5,39 @@
 
 static struct callback_head work_exited; /* all we need is ->next == NULL */
 
+/*
+ * TWA_SIGNAL signaling - use TIF_TASKWORK, if available.
+ */
+static void task_work_signal(struct task_struct *task)
+{
+#ifndef TIF_TASKWORK
+	unsigned long 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);
+	}
+#else
+	set_tsk_thread_flag(task, TIF_TASKWORK);
+	set_notify_resume(task);
+#endif
+}
+
+static inline void clear_tsk_taskwork(struct task_struct *task)
+{
+#ifdef TIF_TASKWORK
+	if (test_tsk_thread_flag(task, TIF_TASKWORK))
+		clear_tsk_thread_flag(task, TIF_TASKWORK);
+#endif
+}
+
 /**
  * task_work_add - ask the @task to execute @work->func()
  * @task: the task which should run the callback
@@ -28,7 +61,6 @@ int
 task_work_add(struct task_struct *task, struct callback_head *work, int notify)
 {
 	struct callback_head *head;
-	unsigned long flags;
 
 	do {
 		head = READ_ONCE(task->task_works);
@@ -42,17 +74,7 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify)
 		set_notify_resume(task);
 		break;
 	case TWA_SIGNAL:
-		/*
-		 * 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);
-		}
+		task_work_signal(task);
 		break;
 	}
 
@@ -110,6 +132,8 @@ void task_work_run(void)
 	struct task_struct *task = current;
 	struct callback_head *work, *head, *next;
 
+	clear_tsk_taskwork(task);
+
 	for (;;) {
 		/*
 		 * work->func() can do task_work_add(), do not set
-- 
2.28.0