Hello Kay.
Kay Sievers wrote:
> > recvmsg() waits until something is queued if udev_monitor->sock is blocking,
>
> Only on a blocking socket, which it isn't by default:
I confirmed using strace utility that Ubuntu 12.04's wait-for-root
(source code shown below) calls recvmsg() using blocking mode.
# strace /usr/lib/initramfs-tools/bin/wait-for-root /dev/sdaX 5
> It's from a time where the socket was still blocking, but we still
> used poll() in calling code. We didn't want to block the caller, hence
> the poll() inside the library.
So, "poll() in udev_monitor_receive_device() does not wait" is a feature?
Then, wait-for-root is responsible for retrying the loop (line 91 to 104) when
udev_monitor_receive_device() returned NULL (without waiting for a "block"
subsystem event) caused by udev_monitor_receive_device() getting a non-"block"
subsystem event at recvmsg(), isn't it?
---------- wait-for-root.c start ----------
1:#include <libudev.h>
2:
3:#include <sys/types.h>
4:#include <sys/stat.h>
5:
6:#include <time.h>
7:#include <stdio.h>
8:#include <limits.h>
9:#include <signal.h>
10:#include <stdlib.h>
11:#include <string.h>
12:#include <unistd.h>
13:
14:
15:static int device_queued (struct udev *udev, const char *path);
16:static int matching_device (struct udev_device *device, const char *path);
17:
18:static void alarm_handler (int signum);
19:
20:
21:int
22:main (int argc,
23: char *argv[])
24:{
25: const char * devpath;
26: char path[PATH_MAX];
27: int timeout;
28: struct udev * udev;
29: struct udev_monitor *udev_monitor;
30: struct stat devstat;
31: struct udev_device * udev_device;
32: const char * type;
33:
34: if (argc != 3) {
35: fprintf (stderr, "Usage: %s DEVICE TIMEOUT\n", argv[0]);
36: exit (2);
37: }
38:
39: devpath = argv[1];
40: if (! strncmp (devpath, "UUID=", 5)) {
41: strcpy (path, "/dev/disk/by-uuid/");
42: strcat (path, devpath + 5);
43: } else if (! strncmp (devpath, "LABEL=", 6)) {
44: strcpy (path, "/dev/disk/by-label/");
45: strcat (path, devpath + 6);
46: } else {
47: strcpy (path, devpath);
48: }
49:
50: timeout = atoi (argv[2]);
51:
52: signal (SIGALRM, alarm_handler);
53: alarm (timeout);
54:
55: /* Connect to the udev monitor first; if we stat() first, the
56: * event might happen between the stat() and the time we actually
57: * get hooked up.
58: */
59: udev = udev_new ();
60: udev_monitor = udev_monitor_new_from_netlink (udev, "udev");
61:
62: udev_monitor_filter_add_match_subsystem_devtype (udev_monitor, "block", NULL);
63: udev_monitor_enable_receiving (udev_monitor);
64:
65: /* If the device is not being processed, check to see whether it
66: * exists already on the filesystem. If this is true, we don't need
67: * to wait for it can obtain the filesystem type by looking up the
68: * udevdb record by major/minor.
69: */
70: if ((! device_queued (udev, devpath))
71: && (stat (path, &devstat) == 0)
72: && S_ISBLK (devstat.st_mode))
73: {
74: udev_device = udev_device_new_from_devnum (udev, 'b', devstat.st_rdev);
75: if (udev_device) {
76: type = udev_device_get_property_value (udev_device, "ID_FS_TYPE");
77: if (type) {
78: printf ("%s\n", type);
79:
80: udev_device_unref (udev_device);
81: goto exit;
82: }
83:
84: udev_device_unref (udev_device);
85: }
86: }
87:
88: /* When the device doesn't exist yet, or is still being processed
89: * by udev, use the monitor socket to wait it to be done.
90: */
91: while ((udev_device = udev_monitor_receive_device (udev_monitor)) != NULL) {
92: if (matching_device (udev_device, devpath)) {
93: type = udev_device_get_property_value (udev_device, "ID_FS_TYPE");
94: if (type) {
95: printf ("%s\n", type);
96:
97: udev_device_unref (udev_device);
98: goto exit;
99: }
100:
101: }
102:
103: udev_device_unref (udev_device);
104: }
105:
106:exit:
107: udev_monitor_unref (udev_monitor);
108: udev_unref (udev);
109:
110: exit (0);
111:}
112:
113:
114:static int
115:device_queued (struct udev *udev,
116: const char * devpath)
117:{
118: struct udev_queue * udev_queue;
119: struct udev_list_entry *queue_entry;
120: int found = 0;
121:
122: udev_queue = udev_queue_new (udev);
123:
124: for (queue_entry = udev_queue_get_queued_list_entry (udev_queue);
125: queue_entry != NULL;
126: queue_entry = udev_list_entry_get_next (queue_entry)) {
127: const char * syspath;
128: struct udev_device *udev_device;
129:
130: syspath = udev_list_entry_get_name (queue_entry);
131: udev_device = udev_device_new_from_syspath (udev, syspath);
132: if (udev_device) {
133: if (matching_device (udev_device, devpath))
134: found = 1;
135:
136: udev_device_unref (udev_device);
137: }
138: }
139:
140: udev_queue_unref (udev_queue);
141:
142: return found;
143:}
144:
145:static int
146:matching_device (struct udev_device *device,
147: const char * path)
148:{
149: const char * devnode;
150: struct udev_list_entry *devlinks_entry;
151:
152: /* Match by name */
153: devnode = udev_device_get_devnode (device);
154: if (devnode && (! strcmp (path, devnode)))
155: return 1;
156:
157: /* Match by UUID */
158: if (! strncmp (path, "UUID=", 5)) {
159: const char *uuid;
160:
161: uuid = udev_device_get_property_value (device, "ID_FS_UUID");
162: if (uuid && (! strcmp (path + 5, uuid)))
163: return 1;
164: }
165:
166: /* Match by LABEL */
167: if (! strncmp (path, "LABEL=", 6)) {
168: const char *label;
169:
170: label = udev_device_get_property_value (device, "ID_FS_LABEL");
171: if (label && (! strcmp (path + 6, label)))
172: return 1;
173: }
174:
175: /* Match by symlink */
176: for (devlinks_entry = udev_device_get_devlinks_list_entry (device);
177: devlinks_entry != NULL;
178: devlinks_entry = udev_list_entry_get_next (devlinks_entry))
179: if (! strcmp (path, udev_list_entry_get_name (devlinks_entry)))
180: return 1;
181:
182: return 0;
183:}
184:
185:
186:static void
187:alarm_handler (int signum)
188:{
189: exit (1);
190:}
---------- wait-for-root.c end ----------
--
To unsubscribe from this list: send the line "unsubscribe linux-hotplug" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
