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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Kevin Easton' <kevin@guarana.org>,
        Daniel Colascione <dancol@google.com>
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        Michal Hocko <mhocko@suse.com>,
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Subject: RE: [PATCH] Add /proc/pid_generation
Thread-Topic: [PATCH] Add /proc/pid_generation
Thread-Index: AQHUglZTZJgLEEWYF0a4KhMuY5AKgaVdNbzw
Date:   Fri, 23 Nov 2018 11:14:17 +0000
Message-ID: <f00df51af0a4491fa7d86f9dc0c07bf6@AcuMS.aculab.com>
References: <20181121201452.77173-1-dancol@google.com>
 <20181122111930.GA7164@ip-172-31-15-78>
In-Reply-To: <20181122111930.GA7164@ip-172-31-15-78>
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From: Kevin Easton
> Sent: 22 November 2018 11:20
> 
> On Wed, Nov 21, 2018 at 12:14:44PM -0800, Daniel Colascione wrote:
> > This change adds a per-pid-namespace 64-bit generation number,
> > incremented on PID rollover, and exposes it via a new proc file
> > /proc/pid_generation. By examining this file before and after /proc
> > enumeration, user code can detect the potential reuse of a PID and
> > restart the task enumeration process, repeating until it gets a
> > coherent snapshot.
> 
> I see downthread this patch has been withdrawn, but nonetheless I'm
> still curious - does this actually solve the problem?
> 
> It seems to me that a PID could be reused within a scan even if the
> generation number remains the same at the beginning and end of a scan:

Why not allocate a 48bit generation number to each 16bit pid?
Then you have a 64bit 'extended-pid' that can be assumed to never be reused.
Provided enough interfaces are enhanced to support 'extended-pid' values
you'll never get reused values.

	David

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