Date: Tue, 7 Apr 2015 20:28:30 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Manfred Spraul <manfred@colorfullife.com>
cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Darren Hart <darren@dvhart.com>,
        Steven Rostedt <rostedt@goodmis.org>, fredrik.markstrom@windriver.com,
        Davidlohr Bueso <davidlohr@hp.com>
Subject: Re: [PATCH 3/3] ipc/mqueue: remove STATE_PENDING
In-Reply-To: <55241851.7060704@colorfullife.com>
Message-ID: <alpine.DEB.2.11.1504072018010.3845@nanos>
References: <1428419030-20030-1-git-send-email-bigeasy@linutronix.de> <1428419030-20030-4-git-send-email-bigeasy@linutronix.de> <55241851.7060704@colorfullife.com>
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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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