On Thu, Mar 28, 2019 at 03:26:19PM +0100, Oleg Nesterov wrote: > 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. I agree. Check my reply to Jann, I already replied to him about this. thanks!
