linux-kernel - [PATCH 3/3] async, kmod: warn on synchronous request

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Date:	Thu, 17 Jan 2013 17:27:13 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Ming Lei <ming.lei@...onical.com>,
	Alex Riesen <raa.lkml@...il.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Jens Axboe <axboe@...nel.dk>,
	USB list <linux-usb@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: [PATCH 3/3] async, kmod: warn on synchronous request_module() from
 async workers

Synchronous requet_module() from an async worker can lead to deadlock
because module init path may invoke async_synchronize_full().  The
async worker waits for request_module() to complete and the module
loading waits for the async task to finish.  This bug happened in the
block layer because of default elevator auto-loading.

Block layer has been updated not to do default elevator auto-loading
and it has been decided to disallow synchronous request_module() from
async workers.

Trigger WARN_ON_ONCE() on synchronous request_module() from async
workers.

For more details, please refer to the following thread.

  http://thread.gmane.org/gmane.linux.kernel/1420814

Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: Alex Riesen <raa.lkml@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
---
Linus, please note that I dropped system_state == SYSTEM_RUNNING
condition from WARN_ON_ONCE() as the deadlock can happen during system
init too - e.g. libata probing block device using async making block
layer try to load default elevator from initramfs.

Thanks.

 kernel/kmod.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 1c317e3..028287e 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -38,6 +38,7 @@
 #include <linux/suspend.h>
 #include <linux/rwsem.h>
 #include <linux/ptrace.h>
+#include <linux/async.h>
 #include <asm/uaccess.h>
 
 #include <trace/events/module.h>
@@ -130,6 +131,14 @@ int __request_module(bool wait, const char *fmt, ...)
 #define MAX_KMOD_CONCURRENT 50	/* Completely arbitrary value - KAO */
 	static int kmod_loop_msg;
 
+	/*
+	 * We don't allow synchronous module loading from async.  Module
+	 * init may invoke async_synchronize_full() which will end up
+	 * waiting for this task which already is waiting for the module
+	 * loading to complete, leading to a deadlock.
+	 */
+	WARN_ON_ONCE(wait && async_current_func(NULL, NULL));
+
 	va_start(args, fmt);
 	ret = vsnprintf(module_name, MODULE_NAME_LEN, fmt, args);
 	va_end(args);
-- 
1.8.0.2

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