Please revert kobject_uevent UMH_NO_WAIT

[Hugh Dickins]

Please revert commit f520360d93cdc37de5d972dac4bf3bdef6a7f6a7
"kobject: don't block for each kobject_uevent".

Tetsuo Handa, running a kernel with CONFIG_DEBUG_PAGEALLOC=y
and CONFIG_UEVENT_HELPER_PATH=/sbin/hotplug, has been hitting RCU
detected CPU stalls: it's been spinning in the loop where do_execve()
counts up the args (but why wasn't fixup_exception working? dunno).

The recent change, switching kobject_uevent_env() from UMH_WAIT_EXEC
to UMH_NO_WAIT, is broken: the exec uses args on the local stack here,
and an env which is kfreed as soon as call_usermodehelper() returns.
It very much needs to wait for the exec to be done.

Or keep the UMH_NO_WAIT, and complicate the code to allocate and free
these resources correctly?  No, as GregKH pointed out when making the
commit, CONFIG_UEVENT_HELPER_PATH="" is a much better optimization -
though some distros are still saying /sbin/hotplug in their .config,
yet with no such binary in their initrd or their root.

Or...

[PATCH] revert kobject_uevent UMH_NO_WAIT

Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
---

 lib/kobject_uevent.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 2.6.30-rc2/lib/kobject_uevent.c	2009-04-08 18:20:24.000000000 +0100
+++ linux/lib/kobject_uevent.c	2009-04-16 20:31:17.000000000 +0100
@@ -258,7 +258,7 @@ int kobject_uevent_env(struct kobject *k
 			goto exit;
 
 		retval = call_usermodehelper(argv[0], argv,
-					     env->envp, UMH_NO_WAIT);
+					     env->envp, UMH_WAIT_EXEC);
 	}
 
 exit:
--
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