linux-kernel - [PATCH 12/16] Reference counting of pid naspaces by pids

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Date:	Fri, 06 Jul 2007 12:10:06 +0400
From:	Pavel Emelianov <xemul@...nvz.org>
To:	Andrew Morton <akpm@...l.org>
CC:	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Serge Hallyn <serue@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux Containers <containers@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kirill Korotaev <dev@...nvz.org>
Subject: [PATCH 12/16] Reference counting of pid naspaces by pids

Getting and putting the pid namespace in alloc_pid() and
free_pid() is too slow. Instead this I get/put the namespace
by the pidmaps. When the pidmap allocates its first pid the
namespace is get, when the pidmap becomes empty - the namespace
is put.

Although pids may live longer than their "fingerpints" in the
pidmaps, this is ok to release the namespace with not yet freed
struct pids, as this pid is not alive and no routines will
use it.

Signed-off-by: Pavel Emelianov <xemul@...nvz.org>

---

 pid.c |   19 +++++++++++++++----
 1 files changed, 15 insertions(+), 4 deletions(-)

diff -upr linux-2.6.22-rc4-mm2.orig/kernel/pid.c linux-2.6.22-rc4-mm2-2/kernel/pid.c
--- linux-2.6.22-rc4-mm2.orig/kernel/pid.c	2007-06-14 12:14:29.000000000 +0400
+++ linux-2.6.22-rc4-mm2-2/kernel/pid.c	2007-07-04 19:00:38.000000000 +0400
@@ -89,16 +95,23 @@ static  __cacheline_aligned_in_smp DEFIN
 
 static fastcall void free_pidmap(struct pid_namespace *pid_ns, int pid)
 {
-	struct pidmap *map = pid_ns->pidmap + pid / BITS_PER_PAGE;
 	int offset = pid & BITS_PER_PAGE_MASK;
+	int map_id = pid / BITS_PER_PAGE;
+	struct pidmap *map = pid_ns->pidmap + map_id;
+	int free_pids;
 
 	clear_bit(offset, map->page);
-	atomic_inc(&map->nr_free);
+	free_pids = atomic_inc_return(&map->nr_free);
+
+	if (map_id == 0)
+		free_pids++;
+	if (free_pids == BITS_PER_PAGE)
+		put_pid_ns(pid_ns);
 }
 
 static int alloc_pidmap(struct pid_namespace *pid_ns)
 {
-	int i, offset, max_scan, pid, last = pid_ns->last_pid;
+	int i, offset, max_scan, pid, last = pid_ns->last_pid, free_pids;
 	struct pidmap *map;
 
 	pid = last + 1;
@@ -126,7 +139,11 @@ static int alloc_pidmap(struct pid_names
 		if (likely(atomic_read(&map->nr_free))) {
 			do {
 				if (!test_and_set_bit(offset, map->page)) {
-					atomic_dec(&map->nr_free);
+					free_pids = atomic_dec_return(
+							&map->nr_free);
+					if (free_pids == BITS_PER_PAGE - 1)
+						get_pid_ns(pid_ns);
+
 					pid_ns->last_pid = pid;
 					return pid;
 				}

-
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