linux-kernel - Re: [PATCH v2 2/2] pidfs: ensure consistent ENOENT/ESRCH reporting

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Message-ID: <20250411-abbitten-caravan-ec53428b33e0@brauner>
Date: Fri, 11 Apr 2025 17:14:36 +0200
From: Christian Brauner <brauner@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, Jeff Layton <jlayton@...nel.org>, 
	Lennart Poettering <lennart@...ttering.net>, Daan De Meyer <daan.j.demeyer@...il.com>, 
	Mike Yuan <me@...dnzj.com>, linux-kernel@...r.kernel.org, 
	Peter Ziljstra <peterz@...radead.org>
Subject: Re: [PATCH v2 2/2] pidfs: ensure consistent ENOENT/ESRCH reporting

On Fri, Apr 11, 2025 at 03:54:45PM +0200, Oleg Nesterov wrote:
> For both patches:
> 
> Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> 
> a minor nit below...
> 
> On 04/11, Christian Brauner wrote:
> >
> >  int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
> >  {
> > -	int err = 0;
> > -
> > -	if (!(flags & PIDFD_THREAD)) {
> > +	scoped_guard(spinlock_irq, &pid->wait_pidfd.lock) {
> > +		/*
> > +		 * If this wasn't a thread-group leader struct pid or
> > +		 * the task already been reaped report ESRCH to
> > +		 * userspace.
> > +		 */
> > +		if (!pid_has_task(pid, PIDTYPE_PID))
> > +			return -ESRCH;
> 
> The "If this wasn't a thread-group leader struct pid" part of the
> comment looks a bit confusing to me, as if pid_has_task(PIDTYPE_PID)
> should return false in this case.

Ok.

> 
> OTOH, perhaps it makes sense to explain scoped_guard(wait_pidfd.lock)?
> Something like "see unhash_process -> wake_up_all(), detach_pid(TGID)
> isn't possible if pid_has_task(PID) succeeds".

I'm verbose. I hope you can live with it:

        /*
         * While holding the pidfd waitqueue lock removing the task
         * linkage for the thread-group leader pid (PIDTYPE_TGID) isn't
         * possible. Thus, if there's still task linkage for PIDTYPE_PID
         * not having thread-group leader linkage for the pid means it
         * wasn't a thread-group leader in the first place.
         */

:)