usermodehelper: implement UMH_KILLABLE

BugLink: http://bugs.launchpad.net/bugs/963685

Implement UMH_KILLABLE, should be used along with UMH_WAIT_EXEC/PROC.
The caller must ensure that subprocess_info->path/etc can not go away
until call_usermodehelper_freeinfo().

call_usermodehelper_exec(UMH_KILLABLE) does
wait_for_completion_killable.  If it fails, it uses
xchg(&sub_info->complete, NULL) to serialize with umh_complete() which
does the same xhcg() to access sub_info->complete.

If call_usermodehelper_exec wins, it can safely return.  umh_complete()
should get NULL and call call_usermodehelper_freeinfo().

Otherwise we know that umh_complete() was already called, in this case
call_usermodehelper_exec() falls back to wait_for_completion() which
should succeed "very soon".

Note: UMH_NO_WAIT == -1 but it obviously should not be used with
UMH_KILLABLE.  We delay the neccessary cleanup to simplify the back
porting.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit d0bd587a80960d7ba7e0c8396e154028c9045c54)

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Andy Whitcroft <andy.whitcroft@canonical.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 8a02da82aa9..846f5695230 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -54,6 +54,8 @@ enum umh_wait {
         UMH_WAIT_PROC = 1,      /* wait for the process to complete */
 };
 
+#define UMH_KILLABLE    4       /* wait for EXEC/PROC killable */
+
 struct subprocess_info {
         struct work_struct work;
         struct completion *complete;