On 03/27, Joel Fernandes wrote: > > Also, based on Kees comment, I think it appears to me that get_pid and > put_pid can race in this way in the original code right? > > get_pid put_pid > > atomic_dec_and_test returns 1 > atomic_inc > kfree > > deref pid /* boom */ > ------------------------------------------------- > > I think get_pid needs to call atomic_inc_not_zero() No. get_pid() should only be used if you already have a reference or you do something like rcu_read_lock(); pid = find_vpid(); get_pid(); rcu_read_lock(); in this case we rely on call_rcu(delayed_put_pid) which drops the initial reference. If put_pid() sees pid->count == 1, then a) nobody else has a reference and b) nobody else can find this pid on rcu-protected lists, so it is safe to free it. Oleg.
