linux-kernel - Re: [PATCH 4/5] pidfd: add CLONE_WAIT

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Message-ID: <20190725114359.GH4707@redhat.com>
Date:   Thu, 25 Jul 2019 13:43:59 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     linux-kernel@...r.kernel.org, arnd@...db.de, ebiederm@...ssion.com,
        keescook@...omium.org, joel@...lfernandes.org, tglx@...utronix.de,
        tj@...nel.org, dhowells@...hat.com, jannh@...gle.com,
        luto@...nel.org, akpm@...ux-foundation.org, cyphar@...har.com,
        torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
        kernel-team@...roid.com, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 4/5] pidfd: add CLONE_WAIT_PID

Or. We can change wait_consider_task() to not clear ->notask_error if
WXXX and the child is PF_WAIT_PID.

This way you can "safely" use wait() without WNOHANG, it won't block if
all the children which can report an even are PF_WAIT_PID.

But I do not understand your use-cases, I have no idea if this can help
or not. Just I think the more discussion is always better when we are
going to add the new API.


On 07/25, Oleg Nesterov wrote:
>
> On 07/25, Christian Brauner wrote:
> >
> > On Thu, Jul 25, 2019 at 12:35:44PM +0200, Oleg Nesterov wrote:
> > >
> > > I have to admit this feature looks a bit exotic to me...
> >
> > It might look like it from the kernels perspective but from the feedback
> > on this when presenting on this userspace has real usecases for this.
> 
> OK...
> 
> but then perhaps we can make PF_WAIT_PID more flexible.
> 
> Say, we can add the new WXXX wait option and change eligible_child()
> 
> 	if ((p->flags & PF_WAIT_PID) && (wo->options & WXXX))
> 		return 0;
> 
> this way the parent can tell waitid() whether the PF_WAIT_PID tasks should
> be filtered or not.
> 
> And if we do this we can even add PR_SET_WAIT_PID/PR_CLR_WAIT_PID instead
> of the new CLONE_ flag.
> 
> Oleg.