Date: Wed, 30 Jan 2008 12:48:01 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: paulmck@linux.vnet.ibm.com, Andrew Morton <akpm@linux-foundation.org>,
       Ingo Molnar <mingo@elte.hu>, Pavel Emelyanov <xemul@openvz.org>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU
Message-ID: <20080130094801.GA143@tv-sign.ru>
References: <20080129164019.GA2060@tv-sign.ru> <m1ir1cgfmm.fsf@ebiederm.dsl.xmission.com> <20080130050048.GH12073@linux.vnet.ibm.com> <m163xbhdq5.fsf@ebiederm.dsl.xmission.com>
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On 01/30, Eric W. Biederman wrote:
>
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
> 
> > On Tue, Jan 29, 2008 at 08:24:17PM -0700, Eric W. Biederman wrote:
> >> 
> >> Using the tasklist_lock to still guarantee we see the list, the entire
> >> list, and exactly the list for proper implementation of kill to
> >> process groups and sessions still seems sane.
> >> 
> >> So let's just remove the guarantee of find_pid being usable with
> >> just the tasklist_lock held.
> >
> > Makes sense to me -- it is totally permissible to hold rcu_read_lock()
> > across update code.  ;-)
> 
> Let me rephrase so it is clear.
> 
> When dealing with pids there is exactly one case where we need
> to take read_lock(&tasklist_lock);

Well, another example is sys_ioprio_set(IOPRIO_WHO_PGRP),

> Posix (and sanely handling corner cases) requires that when we send a
> signal to a process group or a session we have a snapshot in time view
> of the entire group.  In particular this allows us to send SIGKILL to
> every member of the group and to have the entire group die.

but you are right of course. tasklist pins group/session/->tasks.

Oleg.

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