work much harder to reclaim stacks

src/lib9/ffork-Linux.c

@@ -1,23 +1,132 @@
+/*
+ * Is nothing simple?
+ *
+ * We can't free the stack until we've finished executing,
+ * but once we've finished executing, we can't do anything
+ * at all, including call free.  So instead we keep a linked list
+ * of all stacks for all processes, and every few times we try
+ * to allocate a new stack we scan the current stack list for
+ * dead processes and reclaim those stacks.
+ */
+
 #include <u.h>
+#include <sys/types.h>
+#include <sys/wait.h>
 #include <sched.h>
 #include <signal.h>
+#include <errno.h>
 #include <libc.h>
 #include "9proc.h"
 
 int fforkstacksize = 16384;
 
+typedef struct Stack Stack;
+struct Stack
+{
+	Stack *next;
+	Stack *fnext;
+	int pid;
+};
+
+static Lock stacklock;
+static Stack *freestacks;
+static Stack *allstacks;
+static int stackmallocs;
+static void gc(void);
+
+static void*
+mallocstack(void)
+{
+	Stack *p;
+
+	lock(&stacklock);
+top:
+	p = freestacks;
+	if(p)
+		freestacks = p->fnext;
+	else{
+		if(stackmallocs++%1 == 0)
+			gc();
+		if(freestacks)
+			goto top;
+		p = malloc(fforkstacksize);
+		p->next = allstacks;
+		allstacks = p;
+	}
+	if(p)
+		p->pid = 1;
+	unlock(&stacklock);
+	return p;
+}
+
+static void
+gc(void)
+{
+	Stack *p;
+
+	for(p=allstacks; p; p=p->next){
+		if(p->pid > 1)
+		if(kill(p->pid, 0) < 0 && errno == ESRCH){
+			if(0) fprint(2, "reclaim stack from %d\n", p->pid);
+			p->pid = 0;
+		}
+		if(p->pid == 0){
+			p->fnext = freestacks;
+			freestacks = p;
+		}
+	}
+}
+
+static void
+freestack(void *v)
+{
+	Stack *p;
+
+	p = v;
+	if(p == nil)
+		return;
+	lock(&stacklock);
+	p->fnext = freestacks;
+	p->pid = 0;
+	freestacks = p;
+	unlock(&stacklock);
+	return;
+}
+
 static int
 tramp(void *v)
 {
 	void (*fn)(void*), *arg;
 	void **v2;
+	void *p;
 
 	_p9uproc(0);
 	v2 = v;
 	fn = v2[0];
 	arg = v2[1];
+	p = v2[2];
 	free(v2);
 	fn(arg);
+	_exit(0);
+	return 0;
+}
+	
+static int
+trampnowait(void *v)
+{
+	int pid;
+	int cloneflag;
+	void **v2;
+	int *pidp;
+	void *p;
+
+	v2 = v;
+	cloneflag = (int)v2[4];
+	pidp = v2[3];
+	p = v2[2];
+	pid = clone(tramp, p+fforkstacksize-512, cloneflag, v);
+	*pidp = pid;
+	_exit(0);
 	return 0;
 }
 
@@ -26,13 +135,13 @@ ffork(int flags, void (*fn)(void*), void *arg)
 {
 	void **v;
 	char *p;
-	int cloneflag, pid;
+	int cloneflag, pid, thepid, status, nowait;
 
 	_p9uproc(0);
-	p = malloc(fforkstacksize);
-	v = malloc(sizeof(void*)*2);
+	p = mallocstack();
+	v = malloc(sizeof(void*)*5);
 	if(p==nil || v==nil){
-		free(p);
+		freestack(p);
 		free(v);
 		return -1;
 	}
@@ -46,22 +155,34 @@ ffork(int flags, void (*fn)(void*), void *arg)
 		cloneflag |= CLONE_FILES;
 	else
 		flags &= ~RFFDG;
+	nowait = flags&RFNOWAIT;
 	if(!(flags&RFNOWAIT))
 		cloneflag |= SIGCHLD;
 	else
 		flags &= ~RFNOWAIT;
 	if(flags){
 		fprint(2, "unknown rfork flags %x\n", flags);
-		free(p);
+		freestack(p);
 		free(v);
 		return -1;
 	}
 	v[0] = fn;
 	v[1] = arg;
-	pid = clone(tramp, p+fforkstacksize-16, cloneflag, v);
-	if(pid < 0)
-		free(p);
-	return pid;
+	v[2] = p;
+	v[3] = &thepid;
+	v[4] = (void*)cloneflag;
+	thepid = -1;
+	pid = clone(nowait ? trampnowait : tramp, p+fforkstacksize-16, cloneflag, v);
+	if(pid > 0 && nowait){
+		if(wait4(pid, &status, __WALL, 0) < 0)
+			fprint(2, "ffork wait4: %r\n");
+	}else
+		thepid = pid;
+	if(thepid == -1)
+		freestack(p);
+	else
+		((Stack*)p)->pid = thepid;
+	return thepid;
 }
 
 int