[LTP] [Patch v4] fzsync: revoke thread_b if parent hits accidental break

[Li Wang]

We shouldn't rely entirely on the pair->exit flag in tst_fzsync_pair_cleanup()
since there is possible to call tst_brk() at anyplace of thread_a, that will
lead to timeout eventually because of thread_b(tst_fzsync_wait_b) fall into
an infinite(no explicit condition to exit) loop.

Thread_a path trace:
  tst_brk()
    cleanup()
      tst_fzsync_pair_cleanup()
        SAFE_PTHREAD_JOIN(pair->thread_b, NULL)
        #Or pthread_cancel(pair->thread_b)

We fix the problem via a way to kill thread_b with pthread_cancel. With new
wrap_run_b involves enabling thread cancel at the start of the thread B,
then do asynchronous cancellation in tst_fzsync_pair_cleanup if pair->exit
is not being set to 1.

Workaround: [commit 2e74d996: Check recvmmsg exists before entering fuzzy loop]
Signed-off-by: Li Wang <liwang@redhat.com>
Cc: Richard Palethorpe <rpalethorpe@suse.com>
Cc: Cyril Hrubis <chrubis@suse.cz>
---
 include/tst_fuzzy_sync.h | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/tst_fuzzy_sync.h b/include/tst_fuzzy_sync.h
index f9a1947c7..00c74e951 100644
--- a/include/tst_fuzzy_sync.h
+++ b/include/tst_fuzzy_sync.h
@@ -63,6 +63,7 @@
 #include <time.h>
 #include <math.h>
 #include <stdlib.h>
+#include <pthread.h>
 #include "tst_atomic.h"
 #include "tst_timer.h"
 #include "tst_safe_pthread.h"
@@ -218,12 +219,36 @@ static void tst_fzsync_pair_init(struct tst_fzsync_pair *pair)
 static void tst_fzsync_pair_cleanup(struct tst_fzsync_pair *pair)
 {
 	if (pair->thread_b) {
-		tst_atomic_store(1, &pair->exit);
+		/* Revoke thread B if parent hits accidental break */
+		if (!pair->exit) {
+			tst_atomic_store(1, &pair->exit);
+			usleep(100000);
+			pthread_cancel(pair->thread_b);
+			pair->thread_b = 0;
+			return;
+		}
 		SAFE_PTHREAD_JOIN(pair->thread_b, NULL);
 		pair->thread_b = 0;
 	}
 }
 
+/**
+ * Wrap run_b for tst_fzsync_pair_reset to enable pthread cancel
+ * at the start of the thread B.
+ */
+static void *wrap_run_b(void *run_b)
+{
+       void *(*real_run_b)(void *) = run_b;
+
+       if (real_run_b) {
+	       pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL);
+	       pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
+	       (*real_run_b)(NULL);
+       }
+
+       return NULL;
+}
+
 /**
  * Zero some stat fields
  *
@@ -272,7 +297,7 @@ static void tst_fzsync_pair_reset(struct tst_fzsync_pair *pair,
 	pair->b_cntr = 0;
 	pair->exit = 0;
 	if (run_b)
-		SAFE_PTHREAD_CREATE(&pair->thread_b, 0, run_b, 0);
+		SAFE_PTHREAD_CREATE(&pair->thread_b, 0, wrap_run_b, run_b);
 
 	pair->exec_time_start = (float)tst_timeout_remaining();
 }
-- 
2.20.1