Messages in this thread Patch in this message | | | Subject | [PATCH] Fix a race in pid generation that causes pids to be reused immediately. | From | Salman <> | Date | Tue, 08 Jun 2010 23:24:38 -0700 |
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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> --- kernel/pid.c | 11 ++++++++++- 1 files changed, 10 insertions(+), 1 deletions(-)
diff --git a/kernel/pid.c b/kernel/pid.c index e9fd8c1..8cedeab 100644 --- a/kernel/pid.c +++ b/kernel/pid.c @@ -153,8 +153,17 @@ static int alloc_pidmap(struct pid_namespace *pid_ns) if (likely(atomic_read(&map->nr_free))) { do { if (!test_and_set_bit(offset, map->page)) { + int prev; atomic_dec(&map->nr_free); - pid_ns->last_pid = pid; + + do { + prev = last; + last = cmpxchg(&pid_ns->last_pid, + prev, pid); + if (last >= pid) + break; + } while (prev != last); + return pid; } offset = find_next_offset(map, offset);
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