linux-kernel - Re: Lots of bugs with current->state = TASK

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Date:	Thu, 21 Jan 2010 19:12:47 +0100 (CET)
From:	Julia Lawall <julia@...u.dk>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	kernel-janitors <kernel-janitors@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, Greg KH <greg@...ah.com>,
	Andy Whitcroft <apw@...onical.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: Lots of bugs with current->state = TASK_*INTERRUPTIBLE

On Thu, 21 Jan 2010, Steven Rostedt wrote:

> On Thu, 2010-01-21 at 11:47 +0100, Julia Lawall wrote:
> > What about something like the following (drivers/macintosh/adb.c):
> > 
> >         add_wait_queue(&state->wait_queue, &wait);
> >         current->state = TASK_INTERRUPTIBLE;
> > 
> > 	for (;;) {
> > 	        req = state->completed;
> >                 if (req != NULL)
> >                         state->completed = req->next;
> >                 else if (atomic_read(&state->n_pending) == 0)
> >                         ret = -EIO;
> > 		if (req != NULL || ret != 0)
> > 			break;
> > 
> >                 if (file->f_flags & O_NONBLOCK) {
> >                         ret = -EAGAIN;
> >                         break;
> > 		}
> >                 if (signal_pending(current)) {
> >                         ret = -ERESTARTSYS;
> >                         break;
> >                 }
> >                 spin_unlock_irqrestore(&state->lock, flags);
> >                 schedule();
> >         	spin_lock_irqsave(&state->lock, flags);
> >         }
> > 
> >         current->state = TASK_RUNNING;
> >         remove_wait_queue(&state->wait_queue, &wait);
> > 
> > There is a call to schedule eventually after the first current->state 
> > assignment, but it is not right after.
> 
> I looked at this code in a bit more detail. Seems that it does not need
> the set_current_state(), because all activities between the state of the
> task and the variables being checked (state->n_pending, et al) are under
> the state->lock.
> 
> But there should be a comment stating that above the assignment of
> current->state. Something like:
> 
> 	/*
> 	 * No need for the set_current_state() memory barrier since
> 	 * all checks between state and wakeups are done under the
> 	 * state->lock.
> 	 */
> 	current->state = TASK_INTERRUPTIBLE;
> 
> 
> But I'd rather have the author of this code write that.

As far as I can tell, state is something that is local to this driver.  So 
is the point that a lock is taken, or that interrupts are turned off?

julia
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