vault backup: 2025-04-04 23:30:26

Concurrent Systems/ignore this folder/Untitled.md

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+```
+GLOB_MUTEX, W_MUTEX, PR_MUTEX and R_MUTEX semaphores init. at 1
+R a shared register init. at 0
+
+begin_read() :=
+	PRR_MUTEX.down()
+	R_MUTEX.down()
+	R++ # currently active readers
+	if R = 1 then
+		PRW_MUTEX.down()
+		GLOB_MUTEX.down()
+	R_MUTEX.up()
+	PRR_MUTEX.up()
+	return
+
+end_read() :=
+	R_MUTEX.down()
+	R--
+	if R = 0 then
+		GLOB_MUTEX.up()
+		PRW_MUTEX.up()
+	R_MUTEX.up()
+	return
+
+begin_write() :=
+	PRR_MUTEX.down()
+	PRW_MUTEX.down()
+	PRW_MUTEX.up()
+	GLOB_MUTEX.down()
+	return
+
+end_write() :=
+	PRR_MUTEX.up()
+	GLOB_MUTEX.up()
+	return
+```
+
+```
+monitor RW_READERS :=
+	AR, WR, AW, WW, LASTW init at 0
+	condition CR, CW
+
+operation begin_read() :=
+	if WW + AW != 0 then
+		CR.wait()
+		CR.signal()
+	AR++
+
+operation end_read() :=
+	AR--
+	if AR = 0 then
+		CW.signal()
+
+operation begin_write() :=
+	WW++
+	LASTW <- i
+	CW.signal() # wakes eventually other waiting writers *they will return false*
+	if (AR + AW != 0) then
+		CW.wait()
+	if (LASTW != i) then
+		return false
+	AW++
+	WW--
+	return true
+
+operation end_write() :=
+	AW--
+	if WW > 0 then
+		CW.signal()
+	else
+		CR.signal()
+```
+
+```
+CNT initalized to the value I want to initalize the semaphore
+Condition S
+
+up() :=
+	if CNT < 0 then
+		S.signal()
+	CNT++
+
+down() :=
+	CNT--
+	if CNT < 0 then
+		S.wait()
+```
+
+
+```
+p1: (T1)[Rx                   Wx]
+p2: (T2)         [Wx]
+```