mdev: support for serializing hotplug

function                                             old     new   delta
mdev_main                                            503     647    +144

docs/mdev.txt

@@ -95,3 +95,26 @@ properly initialize a device.  Place all such firmware files into the
 filename of the firmware which mdev will load out of /lib/firmware/ and into
 the kernel via the sysfs interface.  The exact filename is hardcoded in the
 kernel, so look there if you need to know how to name the file in userspace.
+
+------------
+ SEQUENCING
+------------
+
+Kernel does not serialize hotplug events. It increments SEQNUM environmental
+variable for each successive hotplug invocation. Normally, mdev doesn't care.
+This may reorder hotplug and hot-unplug events, with typical symptoms of
+device nodes sometimes not created as expected.
+
+However, if /dev/mdev.seq file is found, mdev will compare its
+contents with SEQNUM. It will retry up to two seconds, waiting for them
+to match. If they match exactly (not even trailing '\n' is allowed),
+or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq
+with SEQNUM+1.
+
+IOW: this will serialize concurrent mdev invocations.
+
+If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to
+setting mdev to be the hotplug handler. This writes single '\n' to the file.
+NB: mdev recognizes /dev/mdev.seq consisting of single '\n' characher
+as a special case. IOW: this will not make your first hotplug event
+to stall for two seconds.

util-linux/mdev.c

@@ -374,8 +374,6 @@ static void load_firmware(const char *const firmware, const char *const sysfs_pa
 int mdev_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int mdev_main(int argc UNUSED_PARAM, char **argv)
 {
-	char *action;
-	char *env_path;
 	RESERVE_CONFIG_BUFFER(temp, PATH_MAX + SCRATCH_SIZE);
 
 	/* We can be called as hotplug helper */
@@ -417,8 +415,14 @@ int mdev_main(int argc UNUSED_PARAM, char **argv)
 			ACTION_RECURSE | ACTION_FOLLOWLINKS,
 			fileAction, dirAction, temp, 0);
 	} else {
+		char *seq;
+		char *action;
+		char *env_path;
+		char seqbuf[sizeof(int)*3 + 2];
+		int seqlen = seqlen; /* for compiler */
+
 		/* Hotplug:
-		 * env ACTION=... DEVPATH=... mdev
+		 * env ACTION=... DEVPATH=... [SEQNUM=...] mdev
 		 * ACTION can be "add" or "remove"
 		 * DEVPATH is like "/block/sda" or "/class/input/mice"
 		 */
@@ -427,6 +431,23 @@ int mdev_main(int argc UNUSED_PARAM, char **argv)
 		if (!action || !env_path)
 			bb_show_usage();
 
+		seq = getenv("SEQNUM");
+		if (seq) {
+			int timeout = 2000 / 32;
+			do {
+				seqlen = open_read_close("mdev.seq", seqbuf, sizeof(seqbuf-1));
+				if (seqlen < 0)
+					break;
+				seqbuf[seqlen] = '\0';
+				if (seqbuf[0] == '\n' /* seed file? */
+				 || strcmp(seq, seqbuf) == 0 /* correct idx? */
+				) {
+					break;
+				}
+				usleep(32*1000);
+			} while (--timeout);
+		}
+
 		snprintf(temp, PATH_MAX, "/sys%s", env_path);
 		if (!strcmp(action, "remove"))
 			make_device(temp, 1);
@@ -439,6 +460,10 @@ int mdev_main(int argc UNUSED_PARAM, char **argv)
 					load_firmware(fw, temp);
 			}
 		}
+
+		if (seq && seqlen >= 0) {
+			xopen_xwrite_close("mdev.seq", utoa(xatou(seq) + 1));
+		}
 	}
 
 	if (ENABLE_FEATURE_CLEAN_UP)