[ovs-dev] [PATCH] ofproto-dpif-upcall: Fix a race.

Alex Wang alexw at nicira.com
Tue Mar 25 21:24:11 UTC 2014


Commit 61057e884ca9c(ofproto-dpif-upcall: Slightly simplify
udpif_upcall_handler().) restructured the main loop in
udpif_upcall_handler() and discarded the check for the
'exit_latch' after acquiring the mutex.  This makes it
possible for the following race:

- main thread sets the 'exit_latch' after the handler thread
  checking it.
- main thread acquires the handler thread mutex and signals the
  condition variable of handler thread.
- main thread releases the mutex and 'join' the handler thread.
- handler thread acquires the mutex, finds that n_upcalls is 0
  and waits on the signal of condition variable.
- then OVS will hang forever.

This commit fixes the above issue by adding a check for the
'exit_latch' after acquiring the mutex.

Bug #1217229

Signed-off-by: Alex Wang <alexw at nicira.com>
---
 ofproto/ofproto-dpif-upcall.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/ofproto/ofproto-dpif-upcall.c b/ofproto/ofproto-dpif-upcall.c
index b931ab6..5b5fb6e 100644
--- a/ofproto/ofproto-dpif-upcall.c
+++ b/ofproto/ofproto-dpif-upcall.c
@@ -695,7 +695,10 @@ udpif_upcall_handler(void *arg)
         size_t i;
 
         ovs_mutex_lock(&handler->mutex);
-        if (!handler->n_upcalls) {
+        /* Must check the 'exit_latch' again to make sure the main thread is
+         * not joining on the handler thread. */
+        if (!handler->n_upcalls
+            && !latch_is_set(&handler->udpif->exit_latch)) {
             ovs_mutex_cond_wait(&handler->wake_cond, &handler->mutex);
         }
 
-- 
1.7.9.5




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