From: Matthias Kruk <m@m10k.eu>
Date: Sat, 17 Apr 2021 00:28:00 +0000 (+0900)
Subject: include/sem: Work around busy-waiting in sem_wait()
X-Git-Url: https://git.corax.cc/?a=commitdiff_plain;h=8d0d51960e98e46c8c286a1a32b8edd6f96c0c63;p=toolbox

include/sem: Work around busy-waiting in sem_wait()

The sem_wait() function is implemented using busy-waiting, which is not
good for the environment. This commit adds a call to inotifywait as a
temporary workaround to avoid the busy-waiting.
---

diff --git a/include/sem.sh b/include/sem.sh
index b59ac13..224e771 100644
--- a/include/sem.sh
+++ b/include/sem.sh
@@ -173,16 +173,26 @@ sem_wait() {
 
 	mutex=$(_sem_mutexpath "$name")
 	sem=$(_sem_sempath "$name")
-	passed=0
+	passed=false
 
-	while (( passed == 0 )); do
+	while ! "$passed"; do
 		mutex_lock "$mutex"
 
 		if _sem_dec "$sem"; then
-			passed=1
+			passed=true
 		fi
 
 		mutex_unlock "$mutex"
+
+		# Workaround to prevent busy-waiting. The semaphore
+		# might get increased before we get to the inotifywait,
+		# in which case we'd wait for a whole second, during
+		# which another process might pass the semaphore. This
+		# is not ideal, but to prevent this we'd need something
+		# like pthread_cond_wait().
+		if ! "$passed"; then
+			inotifywait -qq -t 1 "$sem"
+		fi
 	done
 
 	return 0