cls_can: Syntax fix

[lisovros/linux_canprio.git] / kernel / pid.c

diff --git a/kernel/pid.c b/kernel/pid.c
index e9fd8c132d265011e00d75d00e589d83e1d6ce1c..d55c6fb8d087a24a2d462886dfe1fc53bf9deced 100644 (file)
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -122,6 +122,43 @@ static void free_pidmap(struct upid *upid)
        atomic_inc(&map->nr_free);
 }
 
+/*
+ * If we started walking pids at 'base', is 'a' seen before 'b'?
+ */
+static int pid_before(int base, int a, int b)
+{
+       /*
+        * This is the same as saying
+        *
+        * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
+        * and that mapping orders 'a' and 'b' with respect to 'base'.
+        */
+       return (unsigned)(a - base) < (unsigned)(b - base);
+}
+
+/*
+ * We might be racing with someone else trying to set pid_ns->last_pid.
+ * We want the winner to have the "later" value, because if the
+ * "earlier" value prevails, then a pid may get reused immediately.
+ *
+ * Since pids rollover, it is not sufficient to just pick the bigger
+ * value.  We have to consider where we started counting from.
+ *
+ * 'base' is the value of pid_ns->last_pid that we observed when
+ * we started looking for a pid.
+ *
+ * 'pid' is the pid that we eventually found.
+ */
+static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
+{
+       int prev;
+       int last_write = base;
+       do {
+               prev = last_write;
+               last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
+       } while ((prev != last_write) && (pid_before(base, last_write, pid)));
+}
+
 static int alloc_pidmap(struct pid_namespace *pid_ns)
 {
        int i, offset, max_scan, pid, last = pid_ns->last_pid;
@@ -132,7 +169,12 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
                pid = RESERVED_PIDS;
        offset = pid & BITS_PER_PAGE_MASK;
        map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
-       max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
+       /*
+        * If last_pid points into the middle of the map->page we
+        * want to scan this bitmap block twice, the second time
+        * we start with offset == 0 (or RESERVED_PIDS).
+        */
+       max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
        for (i = 0; i <= max_scan; ++i) {
                if (unlikely(!map->page)) {
                        void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
@@ -154,20 +196,12 @@ 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;
+                                       set_last_pid(pid_ns, last, pid);
                                        return pid;
                                }
                                offset = find_next_offset(map, offset);
                                pid = mk_pid(pid_ns, map, offset);
-                       /*
-                        * find_next_offset() found a bit, the pid from it
-                        * is in-bounds, and if we fell back to the last
-                        * bitmap block and the final block was the same
-                        * as the starting point, pid is before last_pid.
-                        */
-                       } while (offset < BITS_PER_PAGE && pid < pid_max &&
-                                       (i != max_scan || pid < last ||
-                                           !((last+1) & BITS_PER_PAGE_MASK)));
+                       } while (offset < BITS_PER_PAGE && pid < pid_max);
                }
                if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
                        ++map;