linux-kernel - Re: [RFC][PATCH 4/7] inotify: Deal with nested sleeps

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Date:	Tue, 5 Aug 2014 10:22:11 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<mingo@...nel.org>, <oleg@...hat.com>,
	<torvalds@...ux-foundation.org>, <tglx@...utronix.de>,
	<ilya.dryomov@...tank.com>, <umgwanakikbuti@...il.com>,
	<linux-kernel@...r.kernel.org>, Eric Paris <eparis@...isplace.org>,
	John McCutchan <john@...nmccutchan.com>,
	Robert Love <rlove@...ve.org>
Subject: Re: [RFC][PATCH 4/7] inotify: Deal with nested sleeps

I don't think this one needs nested sleeps.

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index cc423a3..1ca5888 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -233,15 +233,16 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 	group = file->private_data;
 
 	while (1) {
+		mutex_lock(&group->notification_mutex);
 		prepare_to_wait(&group->notification_waitq, &wait, TASK_INTERRUPTIBLE);
 
-		mutex_lock(&group->notification_mutex);
 		kevent = get_one_event(group, count);
 		mutex_unlock(&group->notification_mutex);
 
 		pr_debug("%s: group=%p kevent=%p\n", __func__, group, kevent);
 
 		if (kevent) {
+			__set_current_state(TASK_RUNNING);
 			ret = PTR_ERR(kevent);
 			if (IS_ERR(kevent))
 				break;



On 08/04/2014 06:30 PM, Peter Zijlstra wrote:
> inotify_read is a wait loop with sleeps in. Wait loops rely on
> task_struct::state and sleeps do too, since that's the only means of
> actually sleeping. Therefore the nested sleeps destroy the wait loop
> state and the wait loop breaks the sleep functions that assume
> TASK_RUNNING (mutex_lock).
> 
> Fix this by using the new woken_wake_function and wait_woken() stuff,
> which registers wakeups in wait and thereby allows shrinking the
> task_state::state changes to the actual sleep part.
> 
> Cc: Eric Paris <eparis@...isplace.org>
> Cc: John McCutchan <john@...nmccutchan.com>
> Cc: Robert Love <rlove@...ve.org>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
>  fs/notify/inotify/inotify_user.c |    9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> --- a/fs/notify/inotify/inotify_user.c
> +++ b/fs/notify/inotify/inotify_user.c
> @@ -227,14 +227,13 @@ static ssize_t inotify_read(struct file
>  	struct fsnotify_event *kevent;
>  	char __user *start;
>  	int ret;
> -	DEFINE_WAIT(wait);
> +	DEFINE_WAIT_FUNC(wait, woken_wake_function);
>  
>  	start = buf;
>  	group = file->private_data;
>  
> +	add_wait_queue(&group->notification_waitq, &wait);
>  	while (1) {
> -		prepare_to_wait(&group->notification_waitq, &wait, TASK_INTERRUPTIBLE);
> -
>  		mutex_lock(&group->notification_mutex);
>  		kevent = get_one_event(group, count);
>  		mutex_unlock(&group->notification_mutex);
> @@ -264,10 +263,10 @@ static ssize_t inotify_read(struct file
>  		if (start != buf)
>  			break;
>  
> -		schedule();
> +		wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
>  	}
> +	remove_wait_queue(&group->notification_waitq, &wait);
>  
> -	finish_wait(&group->notification_waitq, &wait);
>  	if (start != buf && ret != -EFAULT)
>  		ret = buf - start;
>  	return ret;
> 
> 
> --
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