[PATCH] Finish the task 'Replace loginclass mutex with rwlock'

[Tiwei Bie]

> In general I see the change mirrors uifind & friends and seems correct.
> 
> However, I think we can alter the code so that it looks nicer.

Yeah! I have made some modifications to this patch. ;-)

#1. in loginclass_free():

I removed an unnecessary rw_wunlock(&loginclasses_lock) call;

#2. in loginclass_find():

The newly allocated 'lc' could be freed if someone else has created the
loginclass. But I couldn't find a simple way to avoid this. Because we
could not call racct_create() which could sleep while holding the wlock[1].
And this is the code that could sleep in racct_create():

	*racctp = uma_zalloc(racct_zone, M_WAITOK | M_ZERO);

Following is my new patch:

diff --git a/sys/kern/kern_loginclass.c b/sys/kern/kern_loginclass.c
index b20f60b..986c51b 100644
--- a/sys/kern/kern_loginclass.c
+++ b/sys/kern/kern_loginclass.c
@@ -51,13 +51,13 @@ __FBSDID("$FreeBSD$");
 #include <sys/lock.h>
 #include <sys/loginclass.h>
 #include <sys/malloc.h>
-#include <sys/mutex.h>
 #include <sys/types.h>
 #include <sys/priv.h>
 #include <sys/proc.h>
 #include <sys/queue.h>
 #include <sys/racct.h>
 #include <sys/refcount.h>
+#include <sys/rwlock.h>
 #include <sys/sysproto.h>
 #include <sys/systm.h>
 
@@ -68,8 +68,8 @@ LIST_HEAD(, loginclass)	loginclasses;
 /*
  * Lock protecting loginclasses list.
  */
-static struct mtx loginclasses_lock;
-MTX_SYSINIT(loginclasses_init, &loginclasses_lock, "loginclasses lock", MTX_DEF);
+static struct rwlock loginclasses_lock;
+RW_SYSINIT(loginclasses_init, &loginclasses_lock, "loginclasses lock");
 
 void
 loginclass_hold(struct loginclass *lc)
@@ -87,16 +87,30 @@ loginclass_free(struct loginclass *lc)
 	if (old > 1 && atomic_cmpset_int(&lc->lc_refcount, old, old - 1))
 		return;
 
-	mtx_lock(&loginclasses_lock);
+	rw_wlock(&loginclasses_lock);
 	if (refcount_release(&lc->lc_refcount)) {
 		racct_destroy(&lc->lc_racct);
 		LIST_REMOVE(lc, lc_next);
-		mtx_unlock(&loginclasses_lock);
 		free(lc, M_LOGINCLASS);
-
-		return;
 	}
-	mtx_unlock(&loginclasses_lock);
+	rw_wunlock(&loginclasses_lock);
+}
+
+/*
+ * Look up a loginclass struct for the parameter name.
+ * loginclasses_lock must be locked.
+ */
+static struct loginclass *
+loginclass_lookup(const char *name)
+{
+	struct loginclass *lc;
+
+	rw_assert(&loginclasses_lock, RA_LOCKED);
+	LIST_FOREACH(lc, &loginclasses, lc_next)
+		if (strcmp(name, lc->lc_name) == 0)
+			break;
+
+	return (lc);
 }
 
 /*
@@ -109,34 +123,39 @@ loginclass_free(struct loginclass *lc)
 struct loginclass *
 loginclass_find(const char *name)
 {
-	struct loginclass *lc, *newlc;
+	struct loginclass *lc, *oldlc;
 
 	if (name[0] == '\0' || strlen(name) >= MAXLOGNAME)
 		return (NULL);
 
-	newlc = malloc(sizeof(*newlc), M_LOGINCLASS, M_ZERO | M_WAITOK);
-	racct_create(&newlc->lc_racct);
-
-	mtx_lock(&loginclasses_lock);
-	LIST_FOREACH(lc, &loginclasses, lc_next) {
-		if (strcmp(name, lc->lc_name) != 0)
-			continue;
-
-		/* Found loginclass with a matching name? */
-		loginclass_hold(lc);
-		mtx_unlock(&loginclasses_lock);
-		racct_destroy(&newlc->lc_racct);
-		free(newlc, M_LOGINCLASS);
-		return (lc);
+	rw_rlock(&loginclasses_lock);
+	lc = loginclass_lookup(name);
+	if (lc == NULL) {
+		rw_runlock(&loginclasses_lock);
+		lc = malloc(sizeof(*lc), M_LOGINCLASS, M_ZERO | M_WAITOK);
+		racct_create(&lc->lc_racct);
+		rw_wlock(&loginclasses_lock);
+		/*
+		 * There's a chance someone created our loginclass while we
+		 * were in malloc and not holding the lock, so we have to
+		 * make sure we don't insert a duplicate loginclass.
+		 */
+		if ((oldlc = loginclass_lookup(name)) != NULL) {
+			/* Someone else beat us to it. */
+			racct_destroy(&lc->lc_racct);
+			free(lc, M_LOGINCLASS);
+			lc = oldlc;
+		} else {
+			/* Add new loginclass. */
+			strcpy(lc->lc_name, name);
+			refcount_init(&lc->lc_refcount, 1);
+			LIST_INSERT_HEAD(&loginclasses, lc, lc_next);
+		}
 	}
 
-	/* Add new loginclass. */
-	strcpy(newlc->lc_name, name);
-	refcount_init(&newlc->lc_refcount, 1);
-	LIST_INSERT_HEAD(&loginclasses, newlc, lc_next);
-	mtx_unlock(&loginclasses_lock);
-
-	return (newlc);
+	loginclass_hold(lc);
+	rw_unlock(&loginclasses_lock);
+	return (lc);
 }
 
 /*
@@ -222,8 +241,8 @@ loginclass_racct_foreach(void (*callback)(struct racct *racct,
 {
 	struct loginclass *lc;
 
-	mtx_lock(&loginclasses_lock);
+	rw_rlock(&loginclasses_lock);
 	LIST_FOREACH(lc, &loginclasses, lc_next)
 		(callback)(lc->lc_racct, arg2, arg3);
-	mtx_unlock(&loginclasses_lock);
+	rw_runlock(&loginclasses_lock);
 }
-- 
2.1.0

[1] https://www.freebsd.org/cgi/man.cgi?query=rwlock&sektion=9

Tiwei Bie