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Subject: Re: [PATCH v2] Document /proc/pid PID reuse behavior
To:     Pavel Machek <pavel@ucw.cz>, Daniel Colascione <dancol@google.com>
Cc:     linux-kernel@vger.kernel.org, rppt@linux.ibm.com,
        timmurray@google.com, joelaf@google.com, surenb@google.com,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        Prashant Dhamdhere <pdhamdhe@redhat.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
References: <20181031150625.147369-1-dancol@google.com>
 <20181105132205.138695-1-dancol@google.com> <20181119105426.GD28607@amd>
From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Tue, 20 Nov 2018 10:05:21 +0100
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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?
 									Pavel
> 
>