From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date: Mon, 17 Aug 2020 13:45:53 +0000 (+0200)
Subject: 5.8-stable patches
X-Git-Tag: v4.19.140~8
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=c672c74a99ef0748b500ab65b07ad9b0f96cadce;p=thirdparty%2Fkernel%2Fstable-queue.git

5.8-stable patches

added patches:
	task_work-only-grab-task-signal-lock-when-needed.patch
---

diff --git a/queue-5.8/series b/queue-5.8/series
index 971c9fc5604..3a422d9e050 100644
--- a/queue-5.8/series
+++ b/queue-5.8/series
@@ -461,3 +461,4 @@ io_uring-fix-null-pointer-dereference-in-loop_rw_iter.patch
 io_uring-hold-ctx-reference-around-task_work-queue-execute.patch
 io_uring-add-missing-req_f_comp_locked-for-nested-requests.patch
 io_uring-enable-lookup-of-links-holding-inflight-files.patch
+task_work-only-grab-task-signal-lock-when-needed.patch
diff --git a/queue-5.8/task_work-only-grab-task-signal-lock-when-needed.patch b/queue-5.8/task_work-only-grab-task-signal-lock-when-needed.patch
new file mode 100644
index 00000000000..a09d2a55ebb
--- /dev/null
+++ b/queue-5.8/task_work-only-grab-task-signal-lock-when-needed.patch
@@ -0,0 +1,78 @@
+From ebf0d100df0731901c16632f78d78d35f4123bc4 Mon Sep 17 00:00:00 2001
+From: Jens Axboe <axboe@kernel.dk>
+Date: Thu, 13 Aug 2020 09:01:38 -0600
+Subject: task_work: only grab task signal lock when needed
+
+From: Jens Axboe <axboe@kernel.dk>
+
+commit ebf0d100df0731901c16632f78d78d35f4123bc4 upstream.
+
+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>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+
+---
+ kernel/signal.c    |   16 +++++++++++++++-
+ kernel/task_work.c |    8 +++++++-
+ 2 files changed, 22 insertions(+), 2 deletions(-)
+
+--- 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();
+--- a/kernel/task_work.c
++++ b/kernel/task_work.c
+@@ -42,7 +42,13 @@ task_work_add(struct task_struct *task,
+ 		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);