linux-kernel - Re: [PATCH 3/3] ipc/mqueue: remove STATE

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Message-ID: <alpine.DEB.2.11.1504072018010.3845@nanos>
Date:	Tue, 7 Apr 2015 20:28:30 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Manfred Spraul <manfred@...orfullife.com>
cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Darren Hart <darren@...art.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	fredrik.markstrom@...driver.com, Davidlohr Bueso <davidlohr@...com>
Subject: Re: [PATCH 3/3] ipc/mqueue: remove STATE_PENDING

On Tue, 7 Apr 2015, Manfred Spraul wrote:
> On 04/07/2015 05:03 PM, Sebastian Andrzej Siewior wrote:
> > + * spinlock. The same algorithm is used for sysv semaphores, see ipc/sem.c
> > + * for more details.
> No. With your change, ipc/sem.c and ipc/msg.c use different algorithms.
> Please update the comment and describe the new approach:
> 
> Current approach:
> - set pointer to message
> - STATE_PENDING
> - wake_up_process()
> - STATE_READY
>     (now the receiver can continue)
> 
> New approach:
> - set pointer to message
> - get_task_struct
> - STATE_READY
>     (now the receiver can continue, e.g. woken up due to an unrelated
> SIGKILL)
> - wake_up_process()
> - put_task_struct()
> 
> 
> > +		if (r_sender) {
> > +			wake_up_process(r_sender);
> > +			put_task_struct(r_sender);
> > +		}
> >   		ret = 0;
> Could you double-check that it is safe to call wake_up_process on a killed
> and reaped thread, only with a get_task_struct reference?

Yes. It is safe to call wake_up_process() on a dead thread if you hold
a ref.

wake_up_process()
  return try_to_wake_up(p, TASK_NORMAL, 0);
 
try_to_wake_up()
  raw_spin_lock_irqsave(&p->pi_lock, flags);
  if (!(p->state & state))
    goto out;

TASK_NORMAL == (TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE)

That makes try_to_wake_up() a NOOP on a task with state TASK_DEAD. We
have quite some code in the kernel which relies on this.

Thanks,

	tglx
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