linux-kernel - Re: [PATCH 2/4] exit: support non-blocking pidfds

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Message-ID: <20200901161154.GA4386@redhat.com>
Date:   Tue, 1 Sep 2020 18:11:54 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org,
        Christian Brauner <christian@...uner.io>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Aleksa Sarai <cyphar@...har.com>,
        linux-kselftest@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Jens Axboe <axboe@...nel.dk>, linux-api@...r.kernel.org,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 2/4] exit: support non-blocking pidfds

On 08/31, Christian Brauner wrote:
>
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -934,6 +934,7 @@ struct wait_opts {
>  
>  	wait_queue_entry_t		child_wait;
>  	int			notask_error;
> +	int			eagain_error;
>  };
>  
>  static int eligible_pid(struct wait_opts *wo, struct task_struct *p)
> @@ -1461,6 +1462,8 @@ static long do_wait(struct wait_opts *wo)
>  
>  notask:
>  	retval = wo->notask_error;
> +	if (!retval)
> +		retval = wo->eagain_error;
>  	if (!retval && !(wo->wo_flags & WNOHANG)) {
>  		retval = -ERESTARTSYS;

I must have missed something but I don't understand why do we need
the new ->eagain_error and the change in do_wait().

> @@ -1544,6 +1551,11 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
>  	wo.wo_flags	= options;
>  	wo.wo_info	= infop;
>  	wo.wo_rusage	= ru;
> +	wo.eagain_error = 0;
> +	if (f_flags & O_NONBLOCK) {
> +		wo.wo_flags	|= WNOHANG;
> +		wo.eagain_error	= -EAGAIN;
> +	}
>  	ret = do_wait(&wo);

Can't kernel_waitid() simply do

	if (f_flags & O_NONBLOCK)
		wo.wo_flags |= WNOHANG;
	ret = do_wait();
	if (!ret & (f_flags & O_NONBLOCK))
		ret = -EAGAIN;

?

Oleg.