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: Fri, 29 Oct 2010 13:16:49 -0700 Message-ID: <20101029201648.GK2367@linux.vnet.ibm.com> References: <20101029125550.GA3932@swordfish.minsk.epam.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Alexander Viro , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Andrew Morton , Ingo Molnar To: Sergey Senozhatsky Return-path: Received: from e4.ny.us.ibm.com ([32.97.182.144]:48876 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761852Ab0J2UX4 (ORCPT ); Fri, 29 Oct 2010 16:23:56 -0400 Content-Disposition: inline In-Reply-To: <20101029125550.GA3932@swordfish.minsk.epam.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Fri, Oct 29, 2010 at 03:55:50PM +0300, Sergey Senozhatsky wrote: > Commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde "Add RCU check for > find_task_by_vpid()" introduced rcu_lockdep_assert to find_task_by_pid_ns. > Assertion failed in sys_ioprio_get. The patch is fixing assertion > failure in ioprio_set as well. > > =================================================== > [ INFO: suspicious rcu_dereference_check() usage. ] > --------------------------------------------------- > kernel/pid.c:419 invoked rcu_dereference_check() without protection! > > rcu_scheduler_active = 1, debug_locks = 0 > 1 lock held by iotop/4254: > #0: (tasklist_lock){.?.?..}, at: [] sys_ioprio_get+0x22/0x2da > > stack backtrace: > Pid: 4254, comm: iotop Not tainted > Call Trace: > [] lockdep_rcu_dereference+0xaa/0xb2 > [] find_task_by_pid_ns+0x4f/0x68 > [] find_task_by_vpid+0x1d/0x1f > [] sys_ioprio_get+0x50/0x2da > [] system_call_fastpath+0x16/0x1b > > > Signed-off-by: Sergey Senozhatsky > > --- > > diff --git a/fs/ioprio.c b/fs/ioprio.c > index 748cfb9..666343d 100644 > --- a/fs/ioprio.c > +++ b/fs/ioprio.c > @@ -113,8 +113,11 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio) 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? Thanx, Paul > case IOPRIO_WHO_PROCESS: > if (!who) > p = current; > - else > + else { > + rcu_read_lock(); > p = find_task_by_vpid(who); > + rcu_read_unlock(); > + } > if (p) > ret = set_task_ioprio(p, ioprio); > break; > @@ -202,8 +205,11 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who) > case IOPRIO_WHO_PROCESS: > if (!who) > p = current; > - else > + else { > + rcu_read_lock(); > p = find_task_by_vpid(who); > + rcu_read_unlock(); > + } > if (p) > ret = get_task_ioprio(p); > break; > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/