From: Andre Noll <maan@tuebingen.mpg.de>
Date: Fri, 17 Jun 2016 07:18:40 +0000 (+0200)
Subject: ipc: Simplify mutex_try_lock().
X-Git-Tag: v0.1.7~6
X-Git-Url: http://git.tuebingen.mpg.de/?p=dss.git;a=commitdiff_plain;h=03918564cf43cfbd4f873f1b9c6ec3ccd15ede91

ipc: Simplify mutex_try_lock().

There is no need to actually obtain the lock. A single semaphore
operation will do just fine. With sem_op equal to zero and IPC_NOWAIT
the semop() call returns immediately, and the return value tells
whether the semaphore value was zero.

Rename the (static) function to mutex_is_locked() to indicate that
it performs only read-only operations on the semaphore set.
---

diff --git a/ipc.c b/ipc.c
index 478d307..c6f7e6b 100644
--- a/ipc.c
+++ b/ipc.c
@@ -13,6 +13,7 @@
 #include <stddef.h>
 #include <limits.h>
 #include <sys/param.h>
+#include <stdbool.h>
 
 #include "gcc-compat.h"
 #include "str.h"
@@ -310,25 +311,21 @@ static int mutex_lock(int id)
 	return 1;
 }
 
-static int mutex_try_lock(int id)
+static bool mutex_is_locked(int id)
 {
-	struct sembuf sops[2];
+	struct sembuf sops;
 	int ret;
 
 	DSS_DEBUG_LOG(("trying to lock\n"));
 
-	sops[0].sem_num = 0;
-	sops[0].sem_op = 0;
-	sops[0].sem_flg = SEM_UNDO | IPC_NOWAIT;
+	sops.sem_num = 0;
+	sops.sem_op = 0;
+	sops.sem_flg = SEM_UNDO | IPC_NOWAIT;
 
-	sops[1].sem_num = 0;
-	sops[1].sem_op = 1;
-	sops[1].sem_flg = SEM_UNDO | IPC_NOWAIT;
-
-	ret = do_semop(id, sops, 2);
+	ret = do_semop(id, &sops, 1);
 	if (ret < 0)
-		return -ERRNO_TO_DSS_ERROR(errno);
-	return 1;
+		return true;
+	return false;
 }
 
 int lock_dss(char *config_file)
@@ -356,8 +353,5 @@ int get_dss_pid(char *config_file, pid_t *pid)
 		return -E_NOT_RUNNING;
 	if (pid)
 		*pid = ret;
-	ret = mutex_try_lock(semid);
-	if (ret >= 0)
-		return -E_NOT_RUNNING;
-	return 1;
+	return mutex_is_locked(semid)? 1 : -E_NOT_RUNNING;
 }