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From:   Joel Fernandes <joelaf@google.com>
Date:   Tue, 20 Nov 2018 08:37:09 -0800
Message-ID: <CAJWu+orh1imt2iNWQeF+o=uZ-4W=sp4TRzZGi+cO8SectEa0vQ@mail.gmail.com>
Subject: Re: [PATCH v2] Document /proc/pid PID reuse behavior
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     pavel@ucw.cz, Daniel Colascione <dancol@google.com>,
        LKML <linux-kernel@vger.kernel.org>, rppt@linux.ibm.com,
        Tim Murray <timmurray@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>, guro@fb.com,
        rppt@linux.vnet.ibm.com, kirill.shutemov@linux.intel.com,
        dennisszhou@gmail.com, pdhamdhe@redhat.com,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
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On Tue, Nov 20, 2018 at 1:05 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 11/19/18 11:54 AM, Pavel Machek wrote:
> > On Mon 2018-11-05 13:22:05, 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-by: Daniel Colascione <dancol@google.com>
> >> ---
> >>  Documentation/filesystems/proc.txt | 7 +++++++
> >>  1 file changed, 7 insertions(+)
> >>
> >> Moved paragraphed to start of /proc/pid section; added signed-off-by.
> >>
> >> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> >> index 12a5e6e693b6..0b14460f721d 100644
> >> --- a/Documentation/filesystems/proc.txt
> >> +++ b/Documentation/filesystems/proc.txt
> >> @@ -125,6 +125,13 @@ process running on the system, which is named after the process ID (PID).
> >>  The link  self  points  to  the  process reading the file system. Each process
> >>  subdirectory has the entries listed in Table 1-1.
> >>
> >> +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
> >
> > "does not" -> "may not"?
> >
> > We want to leave this unspecified, so that we can change it in future.
>
> Why can't the documentation describe the current implementation, and
> change in the future if the implementation changes? I doubt somebody
> would ever rely on the pid being reused while having the descriptor
> open. How would that make sense?
>

Agreed. I am also of the opinion that this should be documented.

 - Joel