linux-kernel - Re: [RFC][PATCH 3/3] ipc/sem: Rework wakeup scheme

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1316115954.4060.23.camel@twins>
Date:	Thu, 15 Sep 2011 21:45:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC][PATCH 3/3] ipc/sem: Rework wakeup scheme

On Thu, 2011-09-15 at 21:35 +0200, Peter Zijlstra wrote:
> On Thu, 2011-09-15 at 21:32 +0200, Peter Zijlstra wrote:
> > > > +static void wake_up_sem_queue_prepare(struct wake_list_head *wake_list,
> > > >                             struct sem_queue *q, int error)
> > > >   {
> > > > +   struct task_struct *p = ACCESS_ONCE(q->sleeper);
> > > >
> > > > +   get_task_struct(p);
> > > > +   q->status = error;
> > > > +   /*
> > > > +    * implies a full barrier
> > > > +    */
> > > > +   wake_list_add(wake_list, p);
> > > > +   put_task_struct(p);
> > > >   }
> > 
> > > I think the get_task_struct()/put_task_struct is not necessary:
> > > Just do the wake_list_add() before writing q->status:
> > > wake_list_add() is identical to list_add_tail(&q->simple_list, pt).
> > > [except that it contains additional locking, which doesn't matter here]
> 
> OK, I can't read properly.. so the problem with doing the
> wake_list_add() before the write is that the wakeup can actually happen
> before the write in case p already had a wakeup queued.

Ah, but if the wakeup happens early, we return from schedule with -EINTR
and re-acquire the sem_lock and re-test. Since we do this update from
under sem_lock it should serialize and come out all-right,.. right?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/