From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH] rcu_read_lock/unlock protect find_task_by_vpid call Date: Sat, 30 Oct 2010 14:02:44 -0700 Message-ID: <20101030210244.GE2664@linux.vnet.ibm.com> References: <20101029125550.GA3932@swordfish.minsk.epam.com> <20101029201648.GK2367@linux.vnet.ibm.com> <20101030093234.GK3932@swordfish.minsk.epam.com> <201010302214.DDF98906.MSFJOFtFHOLQVO@I-love.SAKURA.ne.jp> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: sergey.senozhatsky@gmail.com, viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, mingo@elte.hu To: Tetsuo Handa Return-path: Content-Disposition: inline In-Reply-To: <201010302214.DDF98906.MSFJOFtFHOLQVO@I-love.SAKURA.ne.jp> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Sat, Oct 30, 2010 at 10:14:23PM +0900, Tetsuo Handa wrote: > Sergey Senozhatsky wrote: > > On (10/29/10 13:16), Paul E. McKenney wrote: > > > Interesting... > > > > > > The task-list lock is read-held at this point, which should mean that > > > the PID mapping cannot change. The lockdep_tasklist_lock_is_held() > > > function does lockdep_is_held(&tasklist_lock), which must therefore > > > only be checking for write-holding the lock. The fix would be to > > > make lockdep_tasklist_lock_is_held() check for either read-holding or > > > write-holding tasklist lock. > > > > > > Or is there some subtle reason that read-holding the tasklist lock is > > > not sufficient? > > This was discussed in the thread at http://kerneltrap.org/mailarchive/linux-kernel/2009/12/10/4517520 . > Quoting from one of posts in that thead http://kerneltrap.org/mailarchive/linux-kernel/2010/2/8/4536388 > > | Usually tasklist gives enough protection, but if copy_process() fails > | it calls free_pid() lockless and does call_rcu(delayed_put_pid(). > | This means, without rcu lock find_pid_ns() can't scan the hash table > | safely. > > And now the patch that adds > > rcu_lockdep_assert(rcu_read_lock_held()); > > was merged in accordance with that comment. > Therefore, I thing below change is not good. > > > Should it be changed to (let's say) > > > > struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns) > > { > > - rcu_lockdep_assert(rcu_read_lock_held()); > > + rcu_lockdep_assert(rcu_read_lock_held() || lockdep_tasklist_lock_is_held()); > > return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID); > > } So we should remove the lockdep_tasklist_lock_is_held() and then apply Sergey's patch, correct? Thanx, Paul