On Tuesday 03 June 2008 16:44, Bernard Blackham wrote:
> Hi,
>
> When I switched from udev+udevd to mdev, booting was much faster and it
> shaved a decent amount from the total image size. However, I've recently
> noticed device nodes sometimes disappearing on boot. The specific
> scenario is:
>
> 1. /dev/ttyS1 exists as a device node in the filesystem on boot - prior
> to the actual device existing, and prior to mdev being called.
>
> 2. echo /bin/mdev > /proc/sys/kernel/hotplug
>
> 3. a module is loaded that provides ttyS1
>
> When this module is loaded, most of the time the existing ttyS1 remains.
> Sometimes however, it is unlinked from the filesystem and is never seen
> again (until mdev -s is run, or next boot, maybe).
>
> I pointed my hotplug to a script to log what mdev was being called as,
> and found that when the module is loaded, there is actually a remove
> event emitted followed by an add event. As there is no locking with
> mdev, sometimes mdev runs the remove after the add and thus the device
> node goes missing.
Despite my solution of having a seq file is considered "lame",
I decided that it's better than nothing.
I would like to ask mdev users to test attached patch.
You need to create /etc/mdev.seq to activate serializing code.
Patch explains this in more details.
--
vda
diff -d -urpN busybox.2/docs/mdev.txt busybox.3/docs/mdev.txt
--- busybox.2/docs/mdev.txt 2008-07-13 01:41:12.000000000 +0200
+++ busybox.3/docs/mdev.txt 2008-07-16 00:32:22.000000000 +0200
@@ -95,3 +95,26 @@ properly initialize a device. Place all
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.
diff -d -urpN busybox.2/util-linux/mdev.c busybox.3/util-linux/mdev.c
--- busybox.2/util-linux/mdev.c 2008-07-15 23:08:44.000000000 +0200
+++ busybox.3/util-linux/mdev.c 2008-07-16 00:16:47.000000000 +0200
@@ -374,8 +374,6 @@ static void load_firmware(const char *co
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, cha
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, cha
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, cha
load_firmware(fw, temp);
}
}
+
+ if (seq && seqlen >= 0) {
+ xopen_xwrite_close("mdev.seq", utoa(xatou(seq) + 1));
+ }
}
if (ENABLE_FEATURE_CLEAN_UP)
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