[PATCH 1/2] kobject: don't block for each kobject_uevent

[Vladimir Davydov]

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 f520360d93c, but it was
wrong (it passed arguments allocated on stack to async thread) so it was
reverted (commit 05f54c13cd0c). It targeted on speeding up the boot
process though.

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
 include/linux/kobject.h |    1 +
 lib/kobject_uevent.c    |   42 ++++++++++++++++++++++++++++++++++++------
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/include/linux/kobject.h b/include/linux/kobject.h
index 926afb6f6b5f..f896a33e8341 100644
--- a/include/linux/kobject.h
+++ b/include/linux/kobject.h
@@ -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];
diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
index 5f72767ddd9b..21c39b32e2aa 100644
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -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);
+		if (!retval)
+			env = NULL;	/* will be freed by cleanup_uevent_env */
 	}
 
 exit:
-- 
1.7.10.4

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