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

[Andrew Morton]

On Sun, 9 Feb 2014 14:56:15 +0400 Vladimir Davydov <vdavydov at 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.

The patches look good to me.  One is kobject (Greg) and the other is
slub (Pekka), so I grabbed them ;) Reviews-and-acks, please?



btw, when referring to commits, please use the form

f520360d93c ("kobject: don't block for each kobject_uevent")

because the same commit can have different hashes in different trees.

(Although I suspect the amount of convenience this provides others
doesn't match the amount of time I spend fixing changelogs!)