linux-kernel - Re: [PATCH] Document /proc/pid PID reuse behavior

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Message-Id: <20181101070856.GB8866@rapoport-lnx>
Date:   Thu, 1 Nov 2018 09:08:57 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     linux-kernel@...r.kernel.org, timmurray@...gle.com,
        joelaf@...gle.com, surenb@...gle.com,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH] Document /proc/pid PID reuse behavior

On Wed, Oct 31, 2018 at 03:06:22PM +0000, Daniel Colascione wrote:
> State explicitly that holding a /proc/pid file descriptor open does
> not reserve the PID. Also note that in the event of PID reuse, these
> open file descriptors refer to the old, now-dead process, and not the
> new one that happens to be named the same numeric PID.

Signed-off is missing.

> ---
>  Documentation/filesystems/proc.txt | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 12a5e6e693b6..567f66a8a23c 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -214,6 +214,14 @@ asynchronous manner and the value may not be very precise. To see a precise
>  snapshot of a moment, you can see /proc/<pid>/smaps file and scan page table.
>  It's slow but very precise.
> 
> +Note that an open a file descriptor to /proc/<pid> or to any of its
> +contained files or subdirectories does not prevent <pid> being reused
> +for some other process in the event that <pid> exits. Operations on
> +open /proc/<pid> file descriptors corresponding to dead processes
> +never act on any new process that the kernel may, through chance, have
> +also assigned the process ID <pid>. Instead, operations on these FDs
> +usually fail with ESRCH.
> +

I'd put this text in the beginning of the section, just before table 1-1.
Otherwise looks good.

It maybe also useful to update 'man 5 proc' (1) as well

[1] https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man5/proc.5

>  Table 1-2: Contents of the status files (as of 4.8)
>  ..............................................................................
>   Field                       Content
> -- 
> 2.19.1.568.g152ad8e336-goog
> 

-- 
Sincerely yours,
Mike.