linux-kernel - [PATCH 1/1 V2] workqueue: remove the del_timer_sync()s in maybe_create

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Date:	Mon, 14 Jul 2014 16:13:21 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	<linux-kernel@...r.kernel.org>
CC:	Tejun Heo <tj@...nel.org>, Lai Jiangshan <laijs@...fujitsu.com>
Subject: [PATCH 1/1 V2] workqueue: remove the del_timer_sync()s in maybe_create_worker()

It is said in the document that the timer which is being
deleted by del_timer_sync() should not be restarted:
  Synchronization rules: Callers must prevent restarting of
  the timer, otherwise this function is meaningless.

Repeating timer may cause the del_timer_sync() spin longer,
or even spin forever in very very very very extreme condition.

It is not considered a bug for me, but it disobeys the document.

To fix it, we need:
  1) don't requeue the mayday timer unless the manager is working
       it is a preparation/cleanup step for the fix. Otherwise the timer
       will repeat forever if we only do the 2).
  2) remove the del_timer_sync()s in maybe_create_worker()
       the timer is still repeating, we should not use del_timer_sync(),
       let the timer dismiss itself.

Change from V1:
  V1 incorrectly stops the timer when !need_to_create_worker(),
  but no one can restart it when needed. In V2, we stops the
  timer only when manager is not working.

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
---
 kernel/workqueue.c |    7 +++----
 1 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 338d418..9c86a64 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1864,10 +1864,11 @@ static void pool_mayday_timeout(unsigned long __pool)
 			send_mayday(work);
 	}
 
+	if (mutex_is_locked(&pool->manager_arb))
+		mod_timer(&pool->mayday_timer, jiffies + MAYDAY_INTERVAL);
+
 	spin_unlock(&pool->lock);
 	spin_unlock_irq(&wq_mayday_lock);
-
-	mod_timer(&pool->mayday_timer, jiffies + MAYDAY_INTERVAL);
 }
 
 /**
@@ -1909,7 +1910,6 @@ restart:
 
 		worker = create_worker(pool);
 		if (worker) {
-			del_timer_sync(&pool->mayday_timer);
 			spin_lock_irq(&pool->lock);
 			start_worker(worker);
 			if (WARN_ON_ONCE(need_to_create_worker(pool)))
@@ -1926,7 +1926,6 @@ restart:
 			break;
 	}
 
-	del_timer_sync(&pool->mayday_timer);
 	spin_lock_irq(&pool->lock);
 	if (need_to_create_worker(pool))
 		goto restart;
-- 
1.7.4.4

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