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SubjectRe: [PATCH 1/2] kobject: don't block for each kobject_uevent
On 02/13/2014 11:53 PM, Andrew Morton wrote:
> On Sun, 9 Feb 2014 14:56:15 +0400 Vladimir Davydov <vdavydov@parallels.com> wrote:
>
>> 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.
> Am not a huge fan of this patch. My test box gets an early oops in
>
> initcalls
> ->...
> ->register_pernet_operations
> ->rtnetlink_net_init
> ->__netlink_kernel_create
> ->sock_create_lite
> ->sock_alloc
> ->new_inode_pseudo
> ->alloc_inode+0xe
>
> I expect that sock_mnt->mnt_sb is null. Or perhaps sb->s_op. Perhaps
> sockfs hasn't mounted yet.
>
> The oops doesn't happen on mainline - it only happens on linux-next.
> So there may be some interaction there, but it may only be timing
> related.
>
> config: http://ozlabs.org/~akpm/stuff/config-akpm2

Oh, that's because I missed that call_usermodehelper_exec() calls
cleanup not only on success, but also on failure resulting in a bunch of
double frees at early boot when khelper hasn't been initialized yet :-(

Please sorry for such a silly mistake. The fixed version is attached.

Thank you.
From 6f08b7e4b59245f4f5b859991931c74e9fcf713a Mon Sep 17 00:00:00 2001
From: Vladimir Davydov <vdavydov@parallels.com>
Date: Fri, 14 Feb 2014 12:32:13 +0400
Subject: [PATCH v2] 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 f520360d93c ("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 05f54c13cd0c ("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>
---
v2: fix env double free on call_usermodehelper_exec failure

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..4e3bd71bd949 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);
+ env = NULL; /* freed by cleanup_uevent_env */
+ }
}

exit:
--
1.7.10.4
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