Commit bcccff93 authored Apr 03, 2014 by Vladimir Davydov Committed by Linus Torvalds Apr 03, 2014

kobject: don't block for each kobject_uevent

Currently kobject_uevent has somewhat unpredictable semantics.  The
point is, since it may call a usermode helper and wait for it to execute
(UMH_WAIT_EXEC), it is impossible to say for sure what lock dependencies
it will introduce for the caller - strictly speaking it depends on what
fs the binary is located on and the set of locks fork may take.  There
are quite a few kobject_uevent's users that do not take this into
account and call it with various mutexes taken, e.g.  rtnl_mutex,
net_mutex, which might potentially lead to a deadlock.

Since there is actually no reason to wait for the usermode helper to
execute there, let's make kobject_uevent start the helper asynchronously
with the aid of the UMH_NO_WAIT flag.

Personally, I'm interested in this, because I really want kobject_uevent
to be called under the slab_mutex in the slub implementation as it used
to be some time ago, because it greatly simplifies synchronization and
automatically fixes a kmemcg-related race.  However, there was a
deadlock detected on an attempt to call kobject_uevent under the
slab_mutex (see https://lkml.org/lkml/2012/1/14/45

), which was reported
to be fixed by releasing the slab_mutex for kobject_uevent.

Unfortunately, there was no information about who exactly blocked on the
slab_mutex causing the usermode helper to stall, neither have I managed
to find this out or reproduce the issue.

BTW, this is not the first attempt to make kobject_uevent use
UMH_NO_WAIT.  Previous one was made by commit f520360d ("kobject:
don't block for each kobject_uevent"), but it was wrong (it passed
arguments allocated on stack to async thread) so it was reverted in
05f54c13 ("Revert "kobject: don't block for each kobject_uevent".").
It targeted on speeding up the boot process though.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Greg KH <greg@kroah.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 5509a5d2

include/linux/kobject.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -119,6 +119,7 @@ struct kobj_type {
		};

		struct kobj_uevent_env {
		char *argv[3];
		char *envp[UEVENT_NUM_ENVP];
		int envp_idx;
		char buf[UEVENT_BUFFER_SIZE];

lib/kobject_uevent.c

+36 −6

Original line number	Diff line number	Diff line
		@@ -124,6 +124,30 @@ static int kobj_usermode_filter(struct kobject *kobj)
		return 0;
		}

		static int init_uevent_argv(struct kobj_uevent_env env, const char subsystem)
		{
		int len;

		len = strlcpy(&env->buf[env->buflen], subsystem,
		sizeof(env->buf) - env->buflen);
		if (len >= (sizeof(env->buf) - env->buflen)) {
		WARN(1, KERN_ERR "init_uevent_argv: buffer size too small\n");
		return -ENOMEM;
		}

		env->argv[0] = uevent_helper;
		env->argv[1] = &env->buf[env->buflen];
		env->argv[2] = NULL;

		env->buflen += len + 1;
		return 0;
		}

		static void cleanup_uevent_env(struct subprocess_info *info)
		{
		kfree(info->data);
		}

		/**
		* kobject_uevent_env - send an uevent with environmental data
		*
		@@ -301,11 +325,8 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,

		/* call uevent_helper, usually only enabled during early boot */
		if (uevent_helper[0] && !kobj_usermode_filter(kobj)) {
		char *argv [3];
		struct subprocess_info *info;

		argv [0] = uevent_helper;
		argv [1] = (char *)subsystem;
		argv [2] = NULL;
		retval = add_uevent_var(env, "HOME=/");
		if (retval)
		goto exit;
		@@ -313,9 +334,18 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
		"PATH=/sbin:/bin:/usr/sbin:/usr/bin");
		if (retval)
		goto exit;
		retval = init_uevent_argv(env, subsystem);
		if (retval)
		goto exit;

		retval = call_usermodehelper(argv[0], argv,
		env->envp, UMH_WAIT_EXEC);
		retval = -ENOMEM;
		info = call_usermodehelper_setup(env->argv[0], env->argv,
		env->envp, GFP_KERNEL,
		NULL, cleanup_uevent_env, env);
		if (info) {
		retval = call_usermodehelper_exec(info, UMH_NO_WAIT);
		env = NULL; /* freed by cleanup_uevent_env */
		}
		}

		exit:

Admin message