[dpdk-stable] patch 'timer: fix race condition' has been queued to LTS release 18.11.1

[Kevin Traynor]

Hi,

FYI, your patch has been queued to LTS release 18.11.1

Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable yet.
It will be pushed if I get no objections before 01/11/19. So please
shout if anyone has objections.

Also note that after the patch there's a diff of the upstream commit vs the
patch applied to the branch. This will indicate if there was any rebasing
needed to apply to the stable branch. If there were code changes for rebasing
(ie: not only metadata diffs), please double check that the rebase was
correctly done.

Thanks.

Kevin Traynor

---
>From 8e49368645c700b04341096cf9c93d0916b314d9 Mon Sep 17 00:00:00 2001
From: Erik Gabriel Carrillo <erik.g.carrillo at intel.com>
Date: Wed, 19 Dec 2018 10:09:34 -0600
Subject: [PATCH] timer: fix race condition

[ upstream commit 7079e29f7f28661b712620f46e6a8514eb0a708a ]

rte_timer_manage() adds expired timers to a "run list", and walks the
list, transitioning each timer from the PENDING to the RUNNING state.
If another lcore resets or stops the timer at precisely this
moment, the timer state would instead be set to CONFIG by that other
lcore, which would cause timer_manage() to skip over it. This is
expected behavior.

However, if a timer expires quickly enough, there exists the
following race condition that causes the timer_manage() routine to
misinterpret a timer in CONFIG state, resulting in lost timers:

- Thread A:
  - starts a timer with rte_timer_reset()
  - the timer is moved to CONFIG state
  - the spinlock associated with the appropriate skiplist is acquired
  - timer is inserted into the skiplist
  - the spinlock is released
- Thread B:
  - executes rte_timer_manage()
  - find above timer as expired, add it to run list
  - walk run list, see above timer still in CONFIG state, unlink it from
    run list and continue on
- Thread A:
  - move timer to PENDING state
  - return from rte_timer_reset()
  - timer is now in PENDING state, but not actually linked into a
    pending list or a run list and will never get processed further
    by rte_timer_manage()

This commit fixes this race condition by only releasing the spinlock
after the timer state has been transitioned from CONFIG to PENDING,
which prevents rte_timer_manage() from seeing an incorrect state.

Fixes: 9b15ba895b9f ("timer: use a skip list")

Signed-off-by: Erik Gabriel Carrillo <erik.g.carrillo at intel.com>
Reviewed-by: Gavin Hu <gavin.hu at arm.com>
---
 lib/librte_timer/rte_timer.c | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/lib/librte_timer/rte_timer.c b/lib/librte_timer/rte_timer.c
index 590488c7e..30c7b0ab4 100644
--- a/lib/librte_timer/rte_timer.c
+++ b/lib/librte_timer/rte_timer.c
@@ -242,22 +242,15 @@ timer_get_prev_entries_for_node(struct rte_timer *tim, unsigned tim_lcore,
 }
 
-/*
- * add in list, lock if needed
+/* call with lock held as necessary
+ * add in list
  * timer must be in config state
  * timer must not be in a list
  */
 static void
-timer_add(struct rte_timer *tim, unsigned tim_lcore, int local_is_locked)
+timer_add(struct rte_timer *tim, unsigned int tim_lcore)
 {
-	unsigned lcore_id = rte_lcore_id();
 	unsigned lvl;
 	struct rte_timer *prev[MAX_SKIPLIST_DEPTH+1];
 
-	/* if timer needs to be scheduled on another core, we need to
-	 * lock the list; if it is on local core, we need to lock if
-	 * we are not called from rte_timer_manage() */
-	if (tim_lcore != lcore_id || !local_is_locked)
-		rte_spinlock_lock(&priv_timer[tim_lcore].list_lock);
-
 	/* find where exactly this element goes in the list of elements
 	 * for each depth. */
@@ -283,7 +276,4 @@ timer_add(struct rte_timer *tim, unsigned tim_lcore, int local_is_locked)
 	priv_timer[tim_lcore].pending_head.expire = priv_timer[tim_lcore].\
 			pending_head.sl_next[0]->expire;
-
-	if (tim_lcore != lcore_id || !local_is_locked)
-		rte_spinlock_unlock(&priv_timer[tim_lcore].list_lock);
 }
 
@@ -380,6 +370,13 @@ __rte_timer_reset(struct rte_timer *tim, uint64_t expire,
 	tim->arg = arg;
 
+	/* if timer needs to be scheduled on another core, we need to
+	 * lock the destination list; if it is on local core, we need to lock if
+	 * we are not called from rte_timer_manage()
+	 */
+	if (tim_lcore != lcore_id || !local_is_locked)
+		rte_spinlock_lock(&priv_timer[tim_lcore].list_lock);
+
 	__TIMER_STAT_ADD(pending, 1);
-	timer_add(tim, tim_lcore, local_is_locked);
+	timer_add(tim, tim_lcore);
 
 	/* update state: as we are in CONFIG state, only us can modify
@@ -390,4 +387,7 @@ __rte_timer_reset(struct rte_timer *tim, uint64_t expire,
 	tim->status.u32 = status.u32;
 
+	if (tim_lcore != lcore_id || !local_is_locked)
+		rte_spinlock_unlock(&priv_timer[tim_lcore].list_lock);
+
 	return 0;
 }
-- 
2.19.0

---
  Diff of the applied patch vs upstream commit (please double-check if non-empty:
---
--- -	2019-01-04 13:23:08.027948323 +0000
+++ 0024-timer-fix-race-condition.patch	2019-01-04 13:23:07.000000000 +0000
@@ -1,8 +1,10 @@
-From 7079e29f7f28661b712620f46e6a8514eb0a708a Mon Sep 17 00:00:00 2001
+From 8e49368645c700b04341096cf9c93d0916b314d9 Mon Sep 17 00:00:00 2001
 From: Erik Gabriel Carrillo <erik.g.carrillo at intel.com>
 Date: Wed, 19 Dec 2018 10:09:34 -0600
 Subject: [PATCH] timer: fix race condition
 
+[ upstream commit 7079e29f7f28661b712620f46e6a8514eb0a708a ]
+
 rte_timer_manage() adds expired timers to a "run list", and walks the
 list, transitioning each timer from the PENDING to the RUNNING state.
 If another lcore resets or stops the timer at precisely this
@@ -37,7 +39,6 @@
 which prevents rte_timer_manage() from seeing an incorrect state.
 
 Fixes: 9b15ba895b9f ("timer: use a skip list")
-Cc: stable at dpdk.org
 
 Signed-off-by: Erik Gabriel Carrillo <erik.g.carrillo at intel.com>
 Reviewed-by: Gavin Hu <gavin.hu at arm.com>