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Date: Wed, 9 Jun 2010 08:17:06 -0400
From: tytso@mit.edu
To: Salman <sqazi@google.com>
Cc: peterz@infradead.org, mingo@elte.hu, akpm@inux-foundation.org,
       linux-kernel@vger.kernel.org, tytso@google.com
Subject: Re: [PATCH] Fix a race in pid generation that causes pids to be
 reused immediately.
Message-ID: <20100609121706.GD6529@thunk.org>
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On Tue, Jun 08, 2010 at 11:24:38PM -0700, Salman wrote:

> A program that repeatedly forks and waits is susceptible to having
> the same pid repeated, especially when it competes with another
> instance of the same program.  This is really bad for bash
> implementation.  Furthermore, many shell scripts assume that pid
> numbers will not be used for some length of time.
> 
> Thanks to Ted Tso for the key ideas of this implementation.
> 
> Signed-off-by: Salman Qazi <sqazi@google.com>

Here's a slightly more succint way of expressing it.  Others will have
decide if it's easier to understand.  (It is for me, but I wrote it.  :-P)

       	       	      	 	      	  - Ted

 kernel/pid.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)
diff --git a/kernel/pid.c b/kernel/pid.c
index e9fd8c1..c51f413 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -154,8 +154,13 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
 					atomic_dec(&map->nr_free);
-					pid_ns->last_pid = pid;
-					return pid;
+					while (1) {
+						i = cmpxchg(&pid_ns->last_pid,
+							    last, pid);
+						if (i == last || i >= pid)
+							return pid;
+						last = i;
+					}
 				}
 				offset = find_next_offset(map, offset);
 				pid = mk_pid(pid_ns, map, offset);


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