linux-kernel - Re: ipv6: tunnel: hang when destroying ipv6 tunnel

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Date:	Sun, 1 Apr 2012 12:21:58 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	levinsasha928@...il.com, oleg@...hat.com
Cc:	eric.dumazet@...il.com, davem@...emloft.net, kuznet@....inr.ac.ru,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org, kaber@...sh.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	davej@...hat.com
Subject: Re: ipv6: tunnel: hang when destroying ipv6 tunnel

Sasha Levin wrote:
> While it seems that 9p is the culprit, I have to point out that this
> bug is easily reproducible, and it happens each time due to a
> call_usermode_helper() call. Other than that 9p behaves perfectly and
> I'd assume that I'd be seeing other things break besides
> call_usermode_helper() related ones.

I think one of below two patches can catch the bug if this is a usermodehelper
related bug. Please try.

----- Patch 1 -----
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 01394b6..3e63319 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -571,7 +571,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 	 * flag, for khelper thread is already waiting for the thread at
 	 * wait_for_completion() in do_fork().
 	 */
-	if (wait != UMH_NO_WAIT && current == kmod_thread_locker) {
+	if (WARN_ON(wait != UMH_NO_WAIT && current == kmod_thread_locker)) {
 		retval = -EBUSY;
 		goto out;
 	}


----- Patch 2 -----
diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 9efeae6..1350670 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -48,10 +48,10 @@ static inline int request_module_nowait(const char *name, ...) { return -ENOSYS;
 struct cred;
 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 */
+#define UMH_NO_WAIT	0x10	/* don't wait at all */
+#define UMH_WAIT_EXEC	0x11	/* wait for the exec, but not the process */
+#define UMH_WAIT_PROC	0x12	/* wait for the process to complete */
+#define UMH_KILLABLE	0x04	/* wait for EXEC/PROC killable */
 
 struct subprocess_info {
 	struct work_struct work;
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 957a7aa..ecfd3d5 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -483,6 +483,18 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 	DECLARE_COMPLETION_ONSTACK(done);
 	int retval = 0;
 
+	if (unlikely(wait == -1 || wait == 0 || wait == 1)) {
+		WARN(1, "Requesting for usermode helper with hardcoded wait "
+		     "flag. Change to use UMH_* symbols and recompile, or "
+		     "this request will fail on Linux 3.4.\n");
+		if (wait == -1)
+			wait = UMH_NO_WAIT;
+		else if (wait == 0)
+			wait = UMH_WAIT_EXEC;
+		else
+			wait = UMH_WAIT_PROC;
+	}
+
 	helper_lock();
 	if (sub_info->path[0] == '\0')
 		goto out;
--
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