linux-kernel - [PATCH 1/3] kthread_worker: Split code for canceling the delayed work timer.

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Message-Id: <20210610133051.15337-2-pmladek@suse.com>
Date:   Thu, 10 Jun 2021 15:30:49 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Martin Liu <liumartin@...gle.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tejun Heo <tj@...nel.org>, minchan@...gle.com,
        davidchao@...gle.com, jenhaochen@...gle.com,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
        Petr Mladek <pmladek@...e.com>
Subject: [PATCH 1/3] kthread_worker: Split code for canceling the delayed work timer.

Simple code refactoring as a preparation step for fixing a race
between  kthread_mod_delayed_work() and
kthread_cancel_delayed_work_sync().

It does not modify the existing behavior.

Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 kernel/kthread.c | 46 +++++++++++++++++++++++++++++-----------------
 1 file changed, 29 insertions(+), 17 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index fe3f2a40d61e..121a0e1fc659 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1092,6 +1092,33 @@ void kthread_flush_work(struct kthread_work *work)
 }
 EXPORT_SYMBOL_GPL(kthread_flush_work);
 
+/*
+ * Make sure that the timer is neither set nor running and could
+ * not manipulate the work list_head any longer.
+ *
+ * The function is called under worker->lock. The lock is temporary
+ * released but the timer can't be set again in the meantime.
+ */
+static void kthread_cancel_delayed_work_timer(struct kthread_work *work,
+					      unsigned long *flags)
+{
+	struct kthread_delayed_work *dwork =
+		container_of(work, struct kthread_delayed_work, work);
+	struct kthread_worker *worker = work->worker;
+
+	/*
+	 * del_timer_sync() must be called to make sure that the timer
+	 * callback is not running. The lock must be temporary released
+	 * to avoid a deadlock with the callback. In the meantime,
+	 * any queuing is blocked by setting the canceling counter.
+	 */
+	work->canceling++;
+	raw_spin_unlock_irqrestore(&worker->lock, *flags);
+	del_timer_sync(&dwork->timer);
+	raw_spin_lock_irqsave(&worker->lock, *flags);
+	work->canceling--;
+}
+
 /*
  * This function removes the work from the worker queue. Also it makes sure
  * that it won't get queued later via the delayed work's timer.
@@ -1106,23 +1133,8 @@ static bool __kthread_cancel_work(struct kthread_work *work, bool is_dwork,
 				  unsigned long *flags)
 {
 	/* Try to cancel the timer if exists. */
-	if (is_dwork) {
-		struct kthread_delayed_work *dwork =
-			container_of(work, struct kthread_delayed_work, work);
-		struct kthread_worker *worker = work->worker;
-
-		/*
-		 * del_timer_sync() must be called to make sure that the timer
-		 * callback is not running. The lock must be temporary released
-		 * to avoid a deadlock with the callback. In the meantime,
-		 * any queuing is blocked by setting the canceling counter.
-		 */
-		work->canceling++;
-		raw_spin_unlock_irqrestore(&worker->lock, *flags);
-		del_timer_sync(&dwork->timer);
-		raw_spin_lock_irqsave(&worker->lock, *flags);
-		work->canceling--;
-	}
+	if (is_dwork)
+		kthread_cancel_delayed_work_timer(work, flags);
 
 	/*
 	 * Try to remove the work from a worker list. It might either
-- 
2.26.2