zfs_dbgmsg_print: make FreeBSD and Linux consistent

FreeBSD was using fprintf(), which might not be signal-safe. Meanwhile,
Linux's locking did not cover the header output. This two quirks are
unrelated, but both have the same response: be like the other one. So
with this commit, both functions are the same except for the names of
their lock and list variables.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes openzfs#16181

Author: robn

module/os/freebsd/zfs/zfs_debug.c

@@ -234,13 +234,29 @@ __dprintf(boolean_t dprint, const char *file, const char *func,
 void
 zfs_dbgmsg_print(const char *tag)
 {
-	zfs_dbgmsg_t *zdm;
+	ssize_t ret __attribute__((unused));
 
-	(void) printf("ZFS_DBGMSG(%s):\n", tag);
 	mutex_enter(&zfs_dbgmsgs_lock);
-	for (zdm = list_head(&zfs_dbgmsgs); zdm;
+
+	/*
+	 * We use write() in this function instead of printf()
+	 * so it is safe to call from a signal handler.
+	 */
+	ret = write(STDOUT_FILENO, "ZFS_DBGMSG(", 11);
+	ret = write(STDOUT_FILENO, tag, strlen(tag));
+	ret = write(STDOUT_FILENO, ") START:\n", 9);
+
+	for (zfs_dbgmsg_t zdm = list_head(&zfs_dbgmsgs); zdm != NULL;
 	    zdm = list_next(&zfs_dbgmsgs, zdm))
-		(void) printf("%s\n", zdm->zdm_msg);
+		ret = write(STDOUT_FILENO, zdm->zdm_msg,
+		    strlen(zdm->zdm_msg));
+		ret = write(STDOUT_FILENO, "\n", 1);
+	}
+
+	ret = write(STDOUT_FILENO, "ZFS_DBGMSG(", 11);
+	ret = write(STDOUT_FILENO, tag, strlen(tag));
+	ret = write(STDOUT_FILENO, ") END\n", 6);
+
 	mutex_exit(&zfs_dbgmsgs_lock);
 }
 #endif /* _KERNEL */

module/os/linux/zfs/zfs_debug.c

@@ -224,6 +224,8 @@ zfs_dbgmsg_print(const char *tag)
 {
 	ssize_t ret __attribute__((unused));
 
+	mutex_enter(&zfs_dbgmsgs.pl_lock);
+
 	/*
 	 * We use write() in this function instead of printf()
 	 * so it is safe to call from a signal handler.
@@ -232,7 +234,6 @@ zfs_dbgmsg_print(const char *tag)
 	ret = write(STDOUT_FILENO, tag, strlen(tag));
 	ret = write(STDOUT_FILENO, ") START:\n", 9);
 
-	mutex_enter(&zfs_dbgmsgs.pl_lock);
 	for (zfs_dbgmsg_t *zdm = list_head(&zfs_dbgmsgs.pl_list); zdm != NULL;
 	    zdm = list_next(&zfs_dbgmsgs.pl_list, zdm)) {
 		ret = write(STDOUT_FILENO, zdm->zdm_msg,