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

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Message-ID: <1264095094.31321.283.camel@gandalf.stny.rr.com>
Date:	Thu, 21 Jan 2010 12:31:34 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Julia Lawall <julia@...u.dk>
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, 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.

-- Steve




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