9os

commit b1ba512aa22afc8e52a4eb0593b5c8da4d5db276
parent 07d157d0fe7503f7dc64e02673fa10f330e5df23
Author: Roberto E. Vargas Caballero <roberto.vargas@midokura.com>
Date:   Wed, 16 Nov 2022 15:47:01 +0100

os9: Fix concurrency problems in sched()

Sched() is a complex function because it has to lock two
processes and the full tasktab data structure. Having 3
locks hold at the same time is dangerous and can drive
to deadlocks easily.

Diffstat:
M
include/os9/os9.h
 | 
1
+
M
src/os9/hosted/arch.c
 | 
14
+++++++++++++-
M
src/os9/proc.c
 | 
37
+++++++++++++++++++++++--------------
M
src/os9/sys.c
 | 
2
+-

4 files changed, 38 insertions(+), 16 deletions(-)
diff --git a/include/os9/os9.h b/include/os9/os9.h
@@ -291,6 +291,7 @@ extern noreturn void trap(Context *ctx);
 extern Ptable *initptable(Task *);
 extern int ictx(Task *, void *);
 extern char *getconf(char *);
+extern void idle(void);
 
 /* dev functions */
 extern void idev(void);
diff --git a/src/os9/hosted/arch.c b/src/os9/hosted/arch.c
@@ -17,10 +17,17 @@ halt(void)
 	abort();
 }
 
+/*
+ * we don't implement context switch in hosted mode
+ * so, this function is going to be called only once
+ * with the entry point of the initial task
+ */
 void
 swtch(Context *ctx)
 {
-	longjmp(recover, 1);
+	void (*fn)(void) = (void (*)(void)) ctx->entry;
+
+	(*fn)();
 }
 
 uint8_t
@@ -59,6 +66,11 @@ outm32(uint32_t val, void *addr)
 	return 0;
 }
 
+void
+idle(void)
+{
+}
+
 int
 ictx(Task *tp, void *fn)
 {
diff --git a/src/os9/proc.c b/src/os9/proc.c
@@ -43,7 +43,8 @@ iproc(void)
 
 	if ((tp = kproc(initfn)) == NULL)
 		panic("init task failed");
-	unlock(&tp->m);
+	tp->affinity = -1;
+	proc = tp;
 }
 
 void
@@ -456,7 +457,7 @@ kproc(void *fn)
 
 	tp->flags = 0;
 
-	tp->state = TRUNNING;
+	tp->state = TREADY;
 	tp->start = SKERNEL;
 
 	if ((tp->kstack = allocb(1)) == NULL)
@@ -472,6 +473,9 @@ err:
 	return NULL;
 }
 
+/*
+ * sched must be called without having any task locked
+ */
 void
 sched(void)
 {
@@ -479,11 +483,15 @@ sched(void)
 	int prio;
 	long long wait, mask;
 
-	lock(&procm);
-	new = proc;
-	if (proc->state == TRUNNING && proc->flags&TLOCK_PREEMP)
-		goto found;
+	if (proc->state == TRUNNING && proc->flags&TLOCK_PREEMP) {
+		unlock(&proc->m);
+		swtch(&proc->ctx);
+	}
+	unlock(&proc->m);
 
+repeat:
+	lock(&procm);
+	new = NULL;
 	prio = -1;
 	wait = -1;
 	mask = 1ul << cpuid;
@@ -492,20 +500,21 @@ sched(void)
 		if (tp->state == TREADY
 		&&  (tp->affinity & mask) != 0
 		&&  tp->prio >= prio
-		&&  (tp->prio != prio || new->wait > wait)) {
-			new = tp;
+		&&  (tp->prio != prio || tp->wait > wait)) {
+			new = proc = tp;
 			prio = tp->prio;
 			wait = tp->wait;
 		}
+		unlock(&tp->m);
+	}
+
+	if (!new) {
+		unlock(&procm);
+		idle();
+		goto repeat;
 	}
 
-found:
-	if (proc->state == TRUNNING)
-		proc->state = TREADY;
-	proc->wait = now;
-	proc = new;
 	proc->state = TRUNNING;
 	unlock(&procm);
-
 	swtch(&proc->ctx);
 }
diff --git a/src/os9/sys.c b/src/os9/sys.c
@@ -109,5 +109,5 @@ isys(void)
 	iconf();
 	icons();
 	iproc();
-	debug();
+	sched();
 }