(Excuse me, but I'm directly registering here because I'm currently having a
trouble with sending mails to / receiving mails from vger.kernel.org domain.)

The system deadlocks when call_usermodehelper(UMH_WAIT_EXEC) request triggered
call_usermodehelper(UMH_WAIT_PROC) request. For example,

  # : > /tmp/dummy
  # chmod 755 /tmp/dummy
  # echo /tmp/dummy > /proc/sys/kernel/hotplug
  # modprobe whatever

generates below output.

INFO: task kworker/u:1:14 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u:1     D 0000000e  6476    14      2 0x00000000
 deee9d68 00000046 30c7da8f 0000000e 30c7da8f 000001b7 0007a4d1 00000000
 d7878d78 dd79e000 c1a258e0 deee63a0 c1a258e0 30cf7f1d 0000000e c1a258e0
 c1a258e0 00000001 0000000e deee63a0 0000000e 0018db76 00000000 ffffffff
Call Trace:
 [<c105ca91>] ? sched_clock_cpu+0x131/0x190
 [<c13946f0>] schedule+0x30/0x50
 [<c1394a45>] schedule_timeout+0x155/0x1c0
 [<c106b6b4>] ? mark_held_locks+0x64/0xf0
 [<c1397012>] ? _raw_spin_unlock_irq+0x22/0x40
 [<c106bb1b>] ? trace_hardirqs_on+0xb/0x10
 [<c1393a92>] wait_for_common+0xd2/0x130
 [<c1036920>] ? try_to_wake_up+0x2b0/0x2b0
 [<c1393b92>] wait_for_completion+0x12/0x20
 [<c103acd8>] do_fork+0xc8/0x280
 [<c13939eb>] ? wait_for_common+0x2b/0x130
 [<c1009d16>] kernel_thread+0x86/0xa0
 [<c104ef30>] ? call_usermodehelper_setup+0x90/0x90
 [<c104ef30>] ? call_usermodehelper_setup+0x90/0x90
 [<c1397e74>] ? common_interrupt+0x34/0x34
 [<c104f2c8>] __call_usermodehelper+0x28/0x90
 [<c104fda8>] process_one_work+0x198/0x3e0
 [<c104fd3c>] ? process_one_work+0x12c/0x3e0
 [<c104f2a0>] ? call_usermodehelper_exec+0x100/0x100
 [<c1051d43>] worker_thread+0x133/0x310
 [<c106bb1b>] ? trace_hardirqs_on+0xb/0x10
 [<c1051c10>] ? manage_workers+0x1e0/0x1e0
 [<c1056cf4>] kthread+0x74/0x80
 [<c1056c80>] ? __init_kthread_worker+0x60/0x60
 [<c1397e7a>] kernel_thread_helper+0x6/0xd
2 locks held by kworker/u:1/14:
 #0:  (khelper){.+.+.+}, at: [<c104fd3c>] process_one_work+0x12c/0x3e0
 #1:  ((&sub_info->work)){+.+.+.}, at: [<c104fd3c>] process_one_work+0x12c/0x3e0

I wrote two patches. Please review.

----- Patch A start -----
[PATCH v3] kmod: Avoid deadlock by recursive kmod call.

The system deadlocks when call_usermodehelper(UMH_WAIT_EXEC) request triggered
call_usermodehelper(UMH_WAIT_PROC) request.

This is because "khelper thread is waiting at wait_for_completion() in
do_fork() since the worker thread was created with CLONE_VFORK flag" and
"the worker thread cannot call complete() because do_execve() is blocked at
UMH_WAIT_PROC request (e.g. request_module() from search_binary_handler())" and
"the khelper thread cannot start processing UMH_WAIT_PROC request because
the khelper thread is waiting at wait_for_completion() in do_fork()".

In order to avoid deadlock, do not try to call wait_for_completion() in
call_usermodehelper_exec() if the worker thread was created by khelper thread
with CLONE_VFORK flag. This bug exists at least since 2.6.10.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 fs/exec.c             |    1 +
 include/linux/sched.h |    3 +++
 kernel/kmod.c         |   13 ++++++++++++-
 3 files changed, 16 insertions(+), 1 deletion(-)

--- linux-3.2.orig/fs/exec.c
+++ linux-3.2/fs/exec.c
@@ -1406,6 +1406,7 @@ int search_binary_handler(struct linux_b
 					fput(bprm->file);
 				bprm->file = NULL;
 				current->did_exec = 1;
+				current->kmod_thread = 0;
 				proc_exec_connector(current);
 				return retval;
 			}
--- linux-3.2.orig/include/linux/sched.h
+++ linux-3.2/include/linux/sched.h
@@ -1302,6 +1302,9 @@ struct task_struct {
 	unsigned sched_reset_on_fork:1;
 	unsigned sched_contributes_to_load:1;
 
+	/* Prevent recursive kmod request. */
+	unsigned kmod_thread:1;
+
 	pid_t pid;
 	pid_t tgid;
 
--- linux-3.2.orig/kernel/kmod.c
+++ linux-3.2/kernel/kmod.c
@@ -189,6 +189,13 @@ fail:
 	do_exit(0);
 }
 
+static int call_helper(void *data)
+{
+	/* Do not trigger recursive kmod call. */
+	current->kmod_thread = 1;
+	return ____call_usermodehelper(data);
+}
+
 void call_usermodehelper_freeinfo(struct subprocess_info *info)
 {
 	if (info->cleanup)
@@ -252,7 +259,7 @@ static void __call_usermodehelper(struct
 		pid = kernel_thread(wait_for_helper, sub_info,
 				    CLONE_FS | CLONE_FILES | SIGCHLD);
 	else
-		pid = kernel_thread(____call_usermodehelper, sub_info,
+		pid = kernel_thread(call_helper, sub_info,
 				    CLONE_VFORK | SIGCHLD);
 
 	switch (wait) {
@@ -428,6 +435,10 @@ int call_usermodehelper_exec(struct subp
 		retval = -EBUSY;
 		goto out;
 	}
+	if (wait != UMH_NO_WAIT && current->kmod_thread) {
+		retval = -EBUSY;
+		goto out;
+	}
 
 	sub_info->complete = &done;
 	sub_info->wait = wait;
----- Patch A end -----

----- Patch B start -----
[PATCH v3] kmod: Avoid deadlock by recursive kmod call.

The system deadlocks when call_usermodehelper(UMH_WAIT_EXEC) request triggered
call_usermodehelper(UMH_WAIT_PROC) request.

This is because "khelper thread is waiting at wait_for_completion() in
do_fork() since the worker thread was created with CLONE_VFORK flag" and
"the worker thread cannot call complete() because do_execve() is blocked at
UMH_WAIT_PROC request (e.g. request_module() from search_binary_handler())" and
"the khelper thread cannot start processing UMH_WAIT_PROC request because
the khelper thread is waiting at wait_for_completion() in do_fork()".

In order to avoid deadlock, do not try to call wait_for_completion() in
call_usermodehelper_exec() if the worker thread is blocking khelper thread.
This bug exists at least since 2.6.10.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 kernel/kmod.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

--- linux-3.2.orig/kernel/kmod.c
+++ linux-3.2/kernel/kmod.c
@@ -43,6 +43,8 @@
 extern int max_threads;
 
 static struct workqueue_struct *khelper_wq;
+static const struct task_struct *khelper_task;
+static const void *khelper_stack;
 
 #define CAP_BSET	(void *)1
 #define CAP_PI		(void *)2
@@ -245,6 +247,8 @@ static void __call_usermodehelper(struct
 	enum umh_wait wait = sub_info->wait;
 	pid_t pid;
 
+	khelper_task = current;
+	khelper_stack = task_stack_page(khelper_task);
 	/* CLONE_VFORK: wait until the usermode helper has execve'd
 	 * successfully We need the data structures to stay around
 	 * until that is done.  */
@@ -402,6 +406,12 @@ void call_usermodehelper_setfns(struct s
 }
 EXPORT_SYMBOL(call_usermodehelper_setfns);
 
+static inline bool object_is_on_khelper_stack(void *obj)
+{
+	return obj && obj >= khelper_stack &&
+		obj < (khelper_stack + THREAD_SIZE);
+}
+
 /**
  * call_usermodehelper_exec - start a usermode application
  * @sub_info: information about the subprocessa
@@ -428,6 +438,11 @@ int call_usermodehelper_exec(struct subp
 		retval = -EBUSY;
 		goto out;
 	}
+	if (wait != UMH_NO_WAIT && current != khelper_task &&
+	    object_is_on_khelper_stack(current->vfork_done)) {
+		retval = -EBUSY;
+		goto out;
+	}
 
 	sub_info->complete = &done;
 	sub_info->wait = wait;
----- Patch B end -----