linux-kernel - Re: [PATCH] fanotify: allow freeze on suspend when waiting for response from userspace

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Date:   Fri, 16 Feb 2018 11:29:13 +0100
From:   Jan Kara <jack@...e.cz>
To:     t.vivek@...sung.com
Cc:     jack@...e.cz, amir73il@...il.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, pankaj.m@...sung.com,
        Kunal Shubham <k.shubham@...sung.com>
Subject: Re: [PATCH] fanotify: allow freeze on suspend when waiting for
 response from userspace

On Fri 16-02-18 15:14:40, t.vivek@...sung.com wrote:
> From: Vivek Trivedi <t.vivek@...sung.com>
> 
> If fanotify userspace response server thread is frozen first,
> it may fail to send response from userspace to kernel space listener.
> In this scenario, fanotify response listener will never get response
> from userepace and fail to suspend.
> 
> Use freeze-friendly wait API to handle this issue.
> 
> Same problem was reported here:
> https://bbs.archlinux.org/viewtopic.php?id=232270
> 
> Freezing of tasks failed after 20.005 seconds
> (1 tasks refusing to freeze, wq_busy=0)
> 
> Backtrace:
> [<c0582f80>] (__schedule) from [<c05835d0>] (schedule+0x4c/0xa4)
> [<c0583584>] (schedule) from [<c01cb648>] (fanotify_handle_event+0x1c8/0x218)
> [<c01cb480>] (fanotify_handle_event) from [<c01c8238>] (fsnotify+0x17c/0x38c)
> [<c01c80bc>] (fsnotify) from [<c02676dc>] (security_file_open+0x88/0x8c)
> [<c0267654>] (security_file_open) from [<c01854b0>] (do_dentry_open+0xc0/0x338)
> [<c01853f0>] (do_dentry_open) from [<c0185a38>] (vfs_open+0x54/0x58)
> [<c01859e4>] (vfs_open) from [<c0195480>] (do_last.isra.10+0x45c/0xcf8)
> [<c0195024>] (do_last.isra.10) from [<c0196140>] (path_openat+0x424/0x600)
> [<c0195d1c>] (path_openat) from [<c0197498>] (do_filp_open+0x3c/0x98)
> [<c019745c>] (do_filp_open) from [<c0186b44>] (do_sys_open+0x120/0x1e4)
> [<c0186a24>] (do_sys_open) from [<c0186c30>] (SyS_open+0x28/0x2c)
> [<c0186c08>] (SyS_open) from [<c0010200>] (__sys_trace_return+0x0/0x20)

Yeah, good catch.

> @@ -63,7 +64,9 @@ static int fanotify_get_response(struct fsnotify_group *group,
>  
>  	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
>  
> -	wait_event(group->fanotify_data.access_waitq, event->response);
> +	while (!event->response)
> +		wait_event_freezable(group->fanotify_data.access_waitq,
> +				     event->response);

But if the process gets a signal while waiting, we will just livelock the
kernel in this loop as wait_event_freezable() will keep returning
ERESTARTSYS. So you need to be a bit more clever here...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR