linux-kernel - Re: [PATCH v2] Document /proc/pid PID reuse behavior

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Message-ID: <CAKOZueu3RQF4hOSg-0LCNVzJsJ=D_yfXg_uun2tqXW8aSkKykw@mail.gmail.com>
Date:   Mon, 19 Nov 2018 08:24:02 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Pavel Machek <pavel@....cz>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Suren Baghdasaryan <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>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2] Document /proc/pid PID reuse behavior

On Mon, Nov 19, 2018 at 2:54 AM, Pavel Machek <pavel@....cz> 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@...gle.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.

No. "Does not". I'm sick and tired of procfs behavior being vague and
unspecified to the point where even kernel developers have an
incorrect mental model of how it all works. With Christian Brauner's
good work in place and hopefully my own work to follow, we're moving
firmly in the direction of procfs handles being struct pid references.
Instead of waffling further, let's just buy into this model and do the
best job we can of making it work well.