Date:	Fri, 3 Apr 2009 14:52:34 -0600
From:	Alex Chiang <achiang@...com>
To:	gregkh@...e.de
Cc:	kaneshige.kenji@...fujitsu.com,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sysfs: don't use global workqueue in
	sysfs_schedule_callback()

Hi Greg,

You're probably still working through your email backlog, but
just wanted to make sure this patch didn't get lost.

Thanks.

* Alex Chiang <achiang@...com>:
> A sysfs attribute using sysfs_schedule_callback() to commit suicide
> may end up calling device_unregister(), which will eventually call
> a driver's ->remove function.
> 
> Drivers may call flush_scheduled_work() in their shutdown routines,
> in which case lockdep will complain with something like the following:
> 
>   =============================================
>   [ INFO: possible recursive locking detected ]
>   2.6.29-rc8-kk #1
>   ---------------------------------------------
>   events/4/56 is trying to acquire lock:
>   (events){--..}, at: [<ffffffff80257fc0>] flush_workqueue+0x0/0xa0
> 
>   but task is already holding lock:
>   (events){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
> 
>   other info that might help us debug this:
>   3 locks held by events/4/56:
>   #0:  (events){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
>   #1:  (&ss->work){--..}, at: [<ffffffff80257648>] run_workqueue+0x108/0x230
>   #2:  (pci_remove_rescan_mutex){--..}, at: [<ffffffff803c10d1>] remove_callback+0x21/0x40
> 
>   stack backtrace:
>   Pid: 56, comm: events/4 Not tainted 2.6.29-rc8-kk #1
>   Call Trace:
>   [<ffffffff8026dfcd>] validate_chain+0xb7d/0x1260
>   [<ffffffff8026eade>] __lock_acquire+0x42e/0xa40
>   [<ffffffff8026f148>] lock_acquire+0x58/0x80
>   [<ffffffff80257fc0>] ? flush_workqueue+0x0/0xa0
>   [<ffffffff8025800d>] flush_workqueue+0x4d/0xa0
>   [<ffffffff80257fc0>] ? flush_workqueue+0x0/0xa0
>   [<ffffffff80258070>] flush_scheduled_work+0x10/0x20
>   [<ffffffffa0144065>] e1000_remove+0x55/0xfe [e1000e]
>   [<ffffffff8033ee30>] ? sysfs_schedule_callback_work+0x0/0x50
>   [<ffffffff803bfeb2>] pci_device_remove+0x32/0x70
>   [<ffffffff80441da9>] __device_release_driver+0x59/0x90
>   [<ffffffff80441edb>] device_release_driver+0x2b/0x40
>   [<ffffffff804419d6>] bus_remove_device+0xa6/0x120
>   [<ffffffff8043e46b>] device_del+0x12b/0x190
>   [<ffffffff8043e4f6>] device_unregister+0x26/0x70
>   [<ffffffff803ba969>] pci_stop_dev+0x49/0x60
>   [<ffffffff803baab0>] pci_remove_bus_device+0x40/0xc0
>   [<ffffffff803c10d9>] remove_callback+0x29/0x40
>   [<ffffffff8033ee4f>] sysfs_schedule_callback_work+0x1f/0x50
>   [<ffffffff8025769a>] run_workqueue+0x15a/0x230
>   [<ffffffff80257648>] ? run_workqueue+0x108/0x230
>   [<ffffffff8025846f>] worker_thread+0x9f/0x100
>   [<ffffffff8025bce0>] ? autoremove_wake_function+0x0/0x40
>   [<ffffffff802583d0>] ? worker_thread+0x0/0x100
>   [<ffffffff8025b89d>] kthread+0x4d/0x80
>   [<ffffffff8020d4ba>] child_rip+0xa/0x20
>   [<ffffffff8020cebc>] ? restore_args+0x0/0x30
>   [<ffffffff8025b850>] ? kthread+0x0/0x80
>   [<ffffffff8020d4b0>] ? child_rip+0x0/0x20
>     
> Although we know that the device_unregister path will never acquire
> a lock that a driver might try to acquire in its ->remove, in general
> we should never attempt to flush a workqueue from within the same
> workqueue, and lockdep rightly complains.
> 
> So as long as sysfs attributes cannot commit suicide directly and we
> are stuck with this callback mechanism, put the sysfs callbacks on
> their own workqueue instead of the global one.
> 
> This has the side benefit that if a suicidal sysfs attribute kicks
> off a long chain of ->remove callbacks, we no longer induce a long
> delay on the global queue.
> 
> Reported-by: Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>
> Tested-by: Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>
> Signed-off-by: Alex Chiang <achiang@...com>
> ---
> This also fixes a missing module_put in the error path introduced
> by sysfs-only-allow-one-scheduled-removal-callback-per-kobj.patch.
> 
> We never destroy the workqueue, but I'm not sure that's a
> problem.
> ---
>  file.c |   12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> ---
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index 289c43a..979e937 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -667,6 +667,7 @@ struct sysfs_schedule_callback_struct {
>  	struct work_struct	work;
>  };
>  
> +static struct workqueue_struct *sysfs_workqueue;
>  static DEFINE_MUTEX(sysfs_workq_mutex);
>  static LIST_HEAD(sysfs_workq);
>  static void sysfs_schedule_callback_work(struct work_struct *work)
> @@ -715,11 +716,20 @@ int sysfs_schedule_callback(struct kobject *kobj, void (*func)(void *),
>  	mutex_lock(&sysfs_workq_mutex);
>  	list_for_each_entry_safe(ss, tmp, &sysfs_workq, workq_list)
>  		if (ss->kobj == kobj) {
> +			module_put(owner);
>  			mutex_unlock(&sysfs_workq_mutex);
>  			return -EAGAIN;
>  		}
>  	mutex_unlock(&sysfs_workq_mutex);
>  
> +	if (sysfs_workqueue == NULL) {
> +		sysfs_workqueue = create_workqueue("sysfsd");
> +		if (sysfs_workqueue == NULL) {
> +			module_put(owner);
> +			return -ENOMEM;
> +		}
> +	}
> +
>  	ss = kmalloc(sizeof(*ss), GFP_KERNEL);
>  	if (!ss) {
>  		module_put(owner);
> @@ -735,7 +745,7 @@ int sysfs_schedule_callback(struct kobject *kobj, void (*func)(void *),
>  	mutex_lock(&sysfs_workq_mutex);
>  	list_add_tail(&ss->workq_list, &sysfs_workq);
>  	mutex_unlock(&sysfs_workq_mutex);
> -	schedule_work(&ss->work);
> +	queue_work(sysfs_workqueue, &ss->work);
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(sysfs_schedule_callback);
> --
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