From 735a6b4380962bc81246c4ae036354208c682898 Mon Sep 17 00:00:00 2001
From: Tomas Mudrunka <tomas@mudrunka.cz>
Date: Tue, 22 Jun 2021 17:23:46 +0200
Subject: [PATCH] Pthread multi

---
 c/pthread_multi.c | 106 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 106 insertions(+)
 create mode 100644 c/pthread_multi.c

diff --git a/c/pthread_multi.c b/c/pthread_multi.c
new file mode 100644
index 0000000..95c581c
--- /dev/null
+++ b/c/pthread_multi.c
@@ -0,0 +1,106 @@
+/*
+ * CFLAGS=-lpthread make pthread_multi
+ */
+
+#include <pthread.h>
+#include <time.h>
+#include <stdio.h>
+#include <stdbool.h>
+
+#define pthread_mutex_swap(a, b) ({ pthread_mutex_t *s; s = (a); a = (b); b = s; })
+
+#define pthread_mutex_lock_two(a,b) pthread_mutex_timedlock_multi_generic((pthread_mutex_t *[2]){(a), (b)}, 2, true, NULL)
+#define pthread_mutex_timedlock_two(a,b,tm) pthread_mutex_timedlock_multi_generic((pthread_mutex_t *[2]){(a), (b)}, 2, true, (tm))
+#define pthread_mutex_trylock_two(a,b) pthread_mutex_timedlock_multi_generic((pthread_mutex_t *[2]){(a), (b)}, 2, false, NULL)
+
+#define pthread_mutex_lock_multi(lcks,cnt) pthread_mutex_timedlock_multi_generic((lcks),(cnt),true,NULL)
+#define pthread_mutex_timedlock_multi(lcks,cnt,tm) pthread_mutex_timedlock_multi_generic((lcks),(cnt),true,(tm))
+#define pthread_mutex_trylock_multi(lcks,cnt) pthread_mutex_timedlock_multi_generic((lcks),(cnt),false,NULL)
+
+/*
+//This is first prototype for two mutexes only, it is useful in order to understand how this all works
+//Currently it was replaced by macro with the same name
+int pthread_mutex_timedlock_two(pthread_mutex_t *a, pthread_mutex_t *b, const struct timespec *restrict abs_timeout) {
+	int ret;
+
+	while(1) {
+		//Wait for the first mutex
+		ret = (abs_timeout==NULL ? pthread_mutex_lock(a) : pthread_mutex_timedlock(a, abs_timeout));
+		if(ret) return ret; //Something is fishy about this mutex. Abort!
+
+		//Try to lock the second mutex
+		ret = pthread_mutex_trylock(b);
+		if(!ret) return ret; //Locked BOTH Hooray!
+
+		//Unlock first if second failed to prevent deadlocks
+		pthread_mutex_unlock(a);
+
+		//Swap mutexes, so we will try to block on the second mutex next time:
+		pthread_mutex_swap(a, b);
+		//printf("Retry!\n");
+	}
+}
+*/
+
+int pthread_mutex_timedlock_multi_generic(pthread_mutex_t **lck, int cnt, bool block, const struct timespec *restrict abs_timeout) {
+	int ret, locked;
+
+	while(1) {
+		//Try to lock all mutexes
+		for(int i = 0; i<cnt; i++) {
+			//Block only on the first one
+			if(block && i==0) {
+				ret = (abs_timeout==NULL ? pthread_mutex_lock(lck[i]) : pthread_mutex_timedlock(lck[i], abs_timeout));
+				if(ret) return ret; //Something went wrong
+				continue;
+			}
+			//Then try if we can lock the rest
+			ret = pthread_mutex_trylock(lck[i]);
+			if(ret) {
+				//Cannot lock this one
+				locked = i;
+				printf("Cannot lock #%d!\n", i);
+				break;
+			}
+			locked = i;
+		}
+
+		//Check if we managed to lock all locks
+		if(locked == cnt) return 0;
+
+		//Unlock all mutexes that we managed to lock so far
+		for(int i = 0;i<locked;i++) {
+			printf("Unlocking #%d\n", i);
+			pthread_mutex_unlock(lck[i]);
+		}
+		if(!block) return ret; //We do not want to block, just trying
+
+		//Try to block on locked mutex in next round
+		pthread_mutex_swap(lck[0],lck[locked]);
+	}
+}
+
+int main() {
+	//Prepare mutex array for tests
+	static pthread_mutex_t la = PTHREAD_MUTEX_INITIALIZER;
+	static pthread_mutex_t lb = PTHREAD_MUTEX_INITIALIZER;
+	static pthread_mutex_t lc = PTHREAD_MUTEX_INITIALIZER;
+	static pthread_mutex_t ld = PTHREAD_MUTEX_INITIALIZER;
+	static pthread_mutex_t *lck[4] = {&la, &lb, &lc, &ld};
+
+	//Set timeout
+	struct  timespec tm;
+	clock_gettime(CLOCK_REALTIME, &tm);
+	tm.tv_sec  += 5;
+
+	//Lock one of the locks for testing
+	pthread_mutex_lock(lck[2]);
+
+	if(!pthread_mutex_timedlock_multi(lck, 4, &tm)) {
+	//if(!pthread_mutex_timedlock_two(&la, lck[2], &tm)) {
+		printf("LOCKED!\n");
+	} else {
+		printf("FAILED!\n");
+	}
+
+}
-- 
2.30.2