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Date:   Wed, 31 Oct 2018 15:06:22 +0000
Message-Id: <20181031150625.147369-1-dancol@google.com>
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Subject: [PATCH] Document /proc/pid PID reuse behavior
From:   Daniel Colascione <dancol@google.com>
To:     linux-kernel@vger.kernel.org
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>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.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.
---
 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.
+
 Table 1-2: Contents of the status files (as of 4.8)
 ..............................................................................
  Field                       Content
-- 
2.19.1.568.g152ad8e336-goog