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

[Vladimir Davydov]

 On 02/13/2014 11:53 PM, Andrew Morton wrote:
> On Sun, 9 Feb 2014 14:56:15 +0400 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> 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.