diff options
| -rw-r--r-- | kernel/pid.c | 39 |
1 files changed, 38 insertions, 1 deletions
diff --git a/kernel/pid.c b/kernel/pid.c index e9fd8c132d26..fbbd5f6b6f2f 100644 --- 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; @@ -154,7 +191,7 @@ 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); |