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Date:   Mon,  5 Nov 2018 13:22:05 +0000
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Subject: [PATCH v2] Document /proc/pid PID reuse behavior
From:   Daniel Colascione <dancol@google.com>
To:     linux-kernel@vger.kernel.org, rppt@linux.ibm.com
Cc:     timmurray@google.com, joelaf@google.com, surenb@google.com,
        Daniel Colascione <dancol@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>,
        Vlastimil Babka <vbabka@suse.cz>,
        "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>
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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
+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.
 
 Table 1-1: Process specific entries in /proc
 ..............................................................................
-- 
2.19.1.930.g4563a0d9d0-goog