[PATCH 3/5] usermodehelper: implement UMH_KILLABLE

[Oleg Nesterov]

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".
---
 include/linux/kmod.h |    1 +
 kernel/kmod.c        |   23 ++++++++++++++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 26187f2..9efeae6 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -51,6 +51,7 @@ struct file;
 #define UMH_NO_WAIT	0	/* don't wait at all */
 #define UMH_WAIT_EXEC	1	/* wait for the exec, but not the process */
 #define UMH_WAIT_PROC	2	/* wait for the process to complete */
+#define UMH_KILLABLE	4	/* wait for EXEC/PROC killable */
 
 struct subprocess_info {
 	struct work_struct work;
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 2443213..7452b4e 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -201,7 +201,12 @@ EXPORT_SYMBOL(call_usermodehelper_freeinfo);
 
 static void umh_complete(struct subprocess_info *sub_info)
 {
-	complete(sub_info->complete);
+	struct completion *done = xchg(&sub_info->complete, NULL);
+
+	if (done)
+		complete(done);
+	else	/* see call_usermodehelper_exec(), we own sub_info */
+		call_usermodehelper_freeinfo(sub_info);
 }
 
 /* Keventd can't block, but this (a child) can. */
@@ -455,14 +460,26 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 	}
 
 	sub_info->complete = &done;
-	sub_info->wait = wait;
+	sub_info->wait = (wait & ~UMH_KILLABLE);
 
 	queue_work(khelper_wq, &sub_info->work);
 	if (wait == UMH_NO_WAIT)	/* task has freed sub_info */
 		goto unlock;
+
+	if (wait & UMH_KILLABLE) {
+		retval = wait_for_completion_killable(&done);
+		if (!retval)
+			goto wait_done;
+
+		/* see umh_complete()->call_usermodehelper_freeinfo() */
+		if (xchg(&sub_info->complete, NULL))
+			goto unlock;
+		/* wait for umh_complete()->complete() in progress... */
+	}
+
 	wait_for_completion(&done);
+wait_done:
 	retval = sub_info->retval;
-
 out:
 	call_usermodehelper_freeinfo(sub_info);
 unlock:
-- 
1.5.5.1


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