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

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Message-ID: <20200901161840.e5eo33ctmq7zavak@wittgenstein>
Date:   Tue, 1 Sep 2020 18:18:40 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.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 Tue, Sep 01, 2020 at 06:11:54PM +0200, Oleg Nesterov wrote:
> 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;
> 
> ?

Heh, indeed, that's even a smaller patch. Will change to that!

Thanks for the review, Oleg!
Christia