linux-kernel - Re: [PATCH] bluetooth rfcomm: Convert to kthread API.

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Message-Id: <20070419161253.301f1b80.akpm@linux-foundation.org>
Date:	Thu, 19 Apr 2007 16:12:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	<containers@...ts.osdl.org>, Oleg Nesterov <oleg@...sign.ru>,
	Christoph Hellwig <hch@...radead.org>,
	<linux-kernel@...r.kernel.org>,
	Marcel Holtmann <marcel@...tmann.org>
Subject: Re: [PATCH] bluetooth rfcomm: Convert to kthread API.

On Thu, 19 Apr 2007 01:58:54 -0600
"Eric W. Biederman" <ebiederm@...ssion.com> wrote:

> From: Eric W. Biederman <ebiederm@...ssion.com>
> 
> This patch starts krfcommd using kthread_run instead of a combination
> of kernel_thread and daemonize making the code slightly simpler
> and more maintainable.

gargh, the more I look at these things, the more I agree with Christoph.

> Cc: Marcel Holtmann <marcel@...tmann.org>
> Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
> ---
>  net/bluetooth/rfcomm/core.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
> index 34f993a..baaad49 100644
> --- a/net/bluetooth/rfcomm/core.c
> +++ b/net/bluetooth/rfcomm/core.c
> @@ -38,6 +38,7 @@
>  #include <linux/net.h>
>  #include <linux/mutex.h>
>  #include <linux/freezer.h>
> +#include <linux/kthread.h>
>  
>  #include <net/sock.h>
>  #include <asm/uaccess.h>
> @@ -1938,7 +1939,6 @@ static int rfcomm_run(void *unused)
>  
>  	atomic_inc(&running);
>  
> -	daemonize("krfcommd");
>  	set_user_nice(current, -10);
>  
>  	BT_DBG("");
> @@ -2058,7 +2058,7 @@ static int __init rfcomm_init(void)
>  
>  	hci_register_cb(&rfcomm_cb);
>  
> -	kernel_thread(rfcomm_run, NULL, CLONE_KERNEL);
> +	kthread_run(rfcomm_run, NULL, "krfcommd");
>  
>  	if (class_create_file(bt_class, &class_attr_rfcomm_dlc) < 0)
>  		BT_ERR("Failed to create RFCOMM info file");

We should remove the file-wide `terminate' and `running' and switch the
thread management over to kthread_run(), kthread_stop() and
kthread_should_stop().

btw, this:

static void rfcomm_worker(void)
{
	BT_DBG("");

	while (!atomic_read(&terminate)) {
		try_to_freeze();

		if (!test_bit(RFCOMM_SCHED_WAKEUP, &rfcomm_event)) {
			/* No pending events. Let's sleep.
			 * Incoming connections and data will wake us up. */
			set_current_state(TASK_INTERRUPTIBLE);
			schedule();
		}

		/* Process stuff */
		clear_bit(RFCOMM_SCHED_WAKEUP, &rfcomm_event);
		rfcomm_process_sessions();
	}
	set_current_state(TASK_RUNNING);
	return;
}

appears to have the classic sleep/wakeup bug: if the wakeup happens after
we tested RFCOMM_SCHED_WAKEUP we will miss it.

Easy fix:

From: Andrew Morton <akpm@...ux-foundation.org>

Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 net/bluetooth/rfcomm/core.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff -puN net/bluetooth/rfcomm/core.c~rfcomm_worker-fix-wakeup-race net/bluetooth/rfcomm/core.c
--- a/net/bluetooth/rfcomm/core.c~rfcomm_worker-fix-wakeup-race
+++ a/net/bluetooth/rfcomm/core.c
@@ -1855,18 +1855,18 @@ static void rfcomm_worker(void)
 	while (!atomic_read(&terminate)) {
 		try_to_freeze();
 
+		set_current_state(TASK_INTERRUPTIBLE);
 		if (!test_bit(RFCOMM_SCHED_WAKEUP, &rfcomm_event)) {
 			/* No pending events. Let's sleep.
 			 * Incoming connections and data will wake us up. */
-			set_current_state(TASK_INTERRUPTIBLE);
 			schedule();
 		}
+		set_current_state(TASK_RUNNING);
 
 		/* Process stuff */
 		clear_bit(RFCOMM_SCHED_WAKEUP, &rfcomm_event);
 		rfcomm_process_sessions();
 	}
-	set_current_state(TASK_RUNNING);
 	return;
 }
 
_


(I think it's safer and saner to always run rfcomm_process_sessions() while
in state TASK_RUNNING, not maybe-in-state-TASK_INTERRUPTIBLE)

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