linux-kernel - Re: [PATCH] fanotify: to differ file access event from different threads

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Date:	Sat, 10 Dec 2011 17:25:38 +0100
From:	Lino Sanfilippo <LinoSanfilippo@....de>
To:	boyd yang <boyd.yang@...il.com>
Cc:	eparis@...hat.com, xiyou.wangcong@...il.com,
	Josef Bacik <josef@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fanotify: to differ file access event from different
 threads

On Tue, Dec 06, 2011 at 09:23:25AM +0800, boyd yang wrote:
> fanotify: to differ file access event from different threads
> When fanotify is monitoring the whole mount point "/", and multiple
> threads iterate the same direcotry, some thread will hang.
> This patch let fanotify to differ access events from different
> threads, prevent fanotify from merging access events from different
> threads.

I could reproduce the problem you described. Actually it may occur each time
when more than one thread of a process is waiting in fanotify_get_response() 
at the same time. This is since we are not prepared to wake up more than one 
waiter: We do 

wait_event(group->fanotify_data.access_waitq, event->response ||
			atomic_read(&group->fanotify_data.bypass_perm));

and after that 
  event->response = 0;

which is the reason that even if we woke up other waiters on the same waitqueue
they may see event->response being already 0, go back to sleep and then possibly 
hang forever.

> It also hide overflow events to reach user space.

This is not part of this patch and also should not be, since overflow
events are supposted to reach user space.

> -	if (event->mask & FAN_ALL_PERM_EVENTS) {
> -		/* if we merged we need to wait on the new event */
> -		if (notify_event)
> -			event = notify_event;
> -		ret = fanotify_get_response_from_access(group, event);
> +	/*if overflow, do not wait for response*/
> +	if (event->mask&FS_Q_OVERFLOW) {
> +		pr_debug("fanotify overflow!\n");
> +	}	else {
> +		if (event->mask & FAN_ALL_PERM_EVENTS) {
> +			/* if we merged we need to wait on the new event */
> +			if (notify_event)
> +				event = notify_event;
> +			ret = fanotify_get_response_from_access(group, event);
> +		}
>  	}

What is this for? All you do is introduce a debug message for no real reason.
However your fix (avoid merging of events from the same process) seems to work.

Regards,
Lino
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